Mitochondrial Associations with Specific Components of the Cortex of Tetrahymena thermophila Nanney & McCoy, 1976: Microtubules and the Epiplasm1

Ultrastructural observations of the cortically-located mitochondria of Tetrahymena thermophila revealed associations not only between the mitochondria and certain of the cortical microtubule bands, but also between the mitochondria and the epiplasm of the cortex. Most of the distal mitochondrial surface is close and parallel to the epiplasm; favorable views show bridge-like structures spanning the 20–10 nm gap between the mitochondrion and the epiplasm. Previous studies have shown that the placement of mitochondria in the cortex appears to be determined by certain of the cortical microtubule bands. This study, however, shows that mitochondrion-microtubule interactions account for only a small proportion of the total mitochondrial area associated with the cortex; the rest is accounted for by the epiplasm. A possible analogue of the spectrin layer of erythrocyte membranes, the epiplasm may be important in helping to arrange the intricately organized components of the ciliate cortex. Its involvement in apparently helping to “moor” mitochondria to their cortical sites is the first suggestion of any role in cell patterning played by the epiplasm.     

Fractal analysis of the egg shell ornamentation in anostracans cysts: a quantitative approach to the morphological variations in Chirocephalus ruffoi

The external ornamentations of the cysts in anostracans are often characterized by crests and ridges. However, their variation cannot be properly quantified by traditional morphometrics. We propose a fractal-based analysis through box-counting to evaluate and compare differences between groups. Box-counting provides a value (fractal dimension, FD) which is proportional to the coverage and space-filling property of a geometrical pattern. Therefore, it is useful to quantify features associated with the spatial organization of the crests. Crests height is moderately correlated with cyst body diameter at individual (R = 0.27; P < 0.0001) and population (R = 0.58; P = 0.05) level. The geometric pattern of the crests is not related to the cyst body size, suggesting that allometric components influencing the spatial distribution of the crests are absent or negligible. There is a faint correlation between crests geometry and height (R = 0.20; P = 0.003). Interestingly, the distribution of the FDs is bimodal, revealing two distinct morphotypes that may be associated with structural constraints, genetics, or environmental components. Taking into account the limited information on the factors shaping the external crests of the cysts, this approach can be useful in evaluating phylogenetic and ecological influences on the final phenotype of the cysts.     

Ultrastructure of the Somatic and Buccal Cortex of the Tetrahymenine Hymenostome Glaucoma chattoni

SYNOPSIS The membranes, epiplasm, and fiber systems are described in the somatic cortex of Glaucoma chattoni strain HZ-1. Kinetodesmal fibers, postciliary and transverse microtubular ribbons, basal microtubules, transverse fibers and transverse accessory material are associated with kinetosomes. Longitudinal microtubular ribbons and mitochondria occur interkinetally. In the buccal cortex, the membranes, epiplasm and fibers of the 3 membranelles, the paroral kinety, the ribbed wall, and the cytostome are described. Comparisons between G. chattoni and other ciliates reveal ultrastructural differences of possible systematic significance. In the somatic cortex of this and other tetrahymenines. Iongitudinal microtubular ribbons and basal microtubules occur concurrently. In the buccal cortex, alveoli are absent in tetrahymenine membranelles. A table is presented of the fiber systems associated with single somatic kinetosomes of various ciliates whose cortical ultrastructure has been studied to date.     

On Paraptychostomum almae N. G., N. Sp., a Commensal Ciliate from the Digestive Tract of Oligochaetes of the Cameroons, in a New Subclass Hysterocinetia

ABSTRACT. Morphological and ultrastructural studies on a new ciliate, Paraptychostomum almae , from the digestive tract of an oligochaete ( Alma emini ) from the Cameroons are carried out. The flattened cell has a large size; its left lateral face bears an anterior thigmotactic zone that includes seven-nine short kinetal segments. The somatic cortex is composed of flattened alveoli, a thin epiplasm and a microfibrillar ecto-endoplasmic boundary. Kineties are made of monokinetids, each particularly characterized by a long anteriorly directed kinetodesmal fiber, and a hyperdivergent postciliary ribbon. The postero-ventral buccal apparatus consists of a short peristome and a deep longitudinal infundibulum. The paroral organelle is a long stichodyad. The three adoral organelles are of different types: ADI and AD3 are of the membranoid type, respectively with two and one rows of ciliated kinetosomes; AD2 is of the peniculus type with six-seven rows of ciliated kinetosomes. A microfibrillar network with nodes arises from all the buccal kinetosomes and extends under the naked wall. Mitochondria are small and numerous and dispersed throughout the whole cell. The existence of an AD2 with more than two rows of kinetosomes warrants the creation of the new genus Paraptychostomum and a new family, Ptychostomatidae. The presence of a distinct ecto-endoplasmic boundary and of somatic kinetids exclusive without transversal dense tractus, hyperdivergent postciliary ribbons, and dispersed numerous mitochondria, added to particularities of the stomatogenesis, allow us to clearly separate hysterocinetians from the scuticociliates and to set up for them the new subclass Hysterocinetia, within the class Oligohymenophorea, with a single new order Hysterocinetida.     

Postnatal development of the respiratory system of the opossum. II. Electron microscopy of the epithelium and pleura.

At birth, the opossum lung is remarkably primitive and consists of a system of branching airways that end in a number of terminal air chambers. From the newborn through the 10 cm stage of development the conducting portion of the lung predominates. The air chambers, which represent portions of the conducting system modified for respiration, are in a constant state of evolution since they are destined to become part of the expanding bronchial system. The airways are devoid of cilia and goblet cells at birth, and are lined by columnar epithelial cells which contain two types of cytoplasmic granules: an electron-dense form and a heterogeneous form. The latter exhibits an electron-dense core surrounded initially by a large halo of flocculent material. This type of granule is not seen beyond the 8 cm stage. The terminal air chambers of the newborn and later stages are lined type I and type II alveolocytes that appear identical to the alveolocytes lining alveoli in the adult. By the 2.5 cm stage, scattered cilia are present in the trachea and bronchi and bands of smooth muscle have differentiated in relation to bronchial epithelium and to proximal areas of the terminal chambers. Citiated cells are separated by ridges composed of light and dark cells which are without cilia and which contain scattered electron-dence granules. Throughout the postnatal period numerous alveolar macrophages and mast cells are noted in relation to the conducting system and pleura. Differentiation of the pleura also occurs during the postnatal period. In the newborn the pleura is simple squamous mesothelium. Later stages develop a thick connective tissue lamina between the pleural mesothelium and lung parenchyma. A large band of elastin is interposed between the mesothelium and underlying bundles of collagen.     

THE FINE STRUCTURE OF CORTICAL COMPONENTS OF PARAMECIUM MULTIMICRONUCLEATUM

The electron microscope was used to study the structure and three dimensional relationships of the components of the body cortex in thin sections of Paramecium multimicronucleatum. Micrographs of sections show that the cortex is covered externally by two closely apposed membranes (together ~250 A thick) constituting the pellicle. Beneath the pellicle the surface of the animal is molded into ridges that form a polygonal ridgework with depressed centers. It is these ridges that give the surface of the organism its characteristic configuration and correspond to the outer fibrillar system of the light microscope image. The outer ends of the trichocysts with their hood-shaped caps are located in the centers of the anterior and posterior ridges of each polygon. The cilia extend singly from the depressed centers of the surface polygons. Each cilium shows two axial filaments with 9 peripheral and parallel filaments embedded in a matrix and the whole surrouned by a thin ciliary membrane. The 9 peripheral filaments are double and these are evenly spaced in a circle around the central pair. The ciliary membrane is continuous with the outer member of the pellicular membrane, whereas the plasma membrane is continuous with the inner member of the pellicular membrane. At the level of the plasma membrane the proximal end of the cilium is continuous with its tube-shaped basal body or kinetosome. The peripheral filaments of the cilium, together with the material of cortical matrix which tends to condense around them, form the sheath of the basal body. The kinetodesma connecting the ciliary kinetosomes (inner fibrillar system of the light microscopist) is composed of a number of discrete fibrils which overlap in a shingle-like fashion. Each striated kinetosomal fibril originates from a ciliary kinetosome and runs parallel to other kinetosomal fibrils arising from posterior kinetosomes of a particular meridional array. Sections at the level of the ciliary kinetosomes reveal an additional fiber system, the infraciliary lattice system, which is separate and distinct from the kinetodesmal system. This system consists of a fibrous network of irregular polygons and runs roughly parallel to the surface of the animal. Mitochondria have a fine structure similar in general features to that described for a number of mammalian cell types, but different in certain details. The structures corresponding to cristae mitochondriales appear as finger-like projections or microvilli extending into the matrix of the organelle from the inner membrane of the paired mitochondrial membrane. The cortical cytoplasm contains also a particulate component and a system of vesicles respectively comparable to the nucleoprotein particles and to the endoplasmic reticulum described in various metazoan cell types. An accessory kinetosome has been observed in oblique sections of a number of non-dividing specimens slightly removed from the ciliary kinetosome and on the same meridional line as the cilia and trichocysts. Its position corresponds to the location of the kinetosome of the newly formed cilium in animals selected as being in the approaching fission stage of the life cycle.     

Ultrastructure of the Somatic Cortex of the Gymnostome Ciliate Spathidium spathula (O. F. M.)1

The somatic cortex of Spathidium spathula is described ultrastructurally. The pellicle consists of an outer membrane and an underlying alveolar system. Numerous membrane-bound mucocysts and spherical electron-opaque bodies have similar circular sites of attachment to the outer membrane. Below these are a microfibrillar zone and an underlying region of rough ER. Mitochondria are lined up under the rough ER in longitudinal cortical ridges. Parasomal sacs are found near the basal bodies and are associated with cytoplasmic membranous sacs. Various microtubular and fiber systems are associated with single basal bodies: (1) a short kinetodesmal fiber; (2) two transverse microtubular ribbons and a transverse fiber; (3) a postciliary microtubular ribbon, initially sandwiched by two fibers, which gives rise to longitudinal subpellicular microtubules extending posteriorly for a distance of some four or five basal bodies; and (4) a system of overlapping subkinetal microtubules. A three-dimensional reconstruction is included. The somatic cortex of S spathula is similar to that reported for other Haptorida of the ciliate subclass Gymnostomata.     

Scanning Electron Microscopy of the Cortex of the Ciliate Stentor coeruleus. A View from the Inside*

SYNOPSIS. A microdissection procedure was developed which permits the viewing of the inside surface of the cortex of Stentor coeruleus with scanning electron microscopy. Parallel bands of myonemes cover the entire inner surface of the cortex. The myonemes of the stalk region are ribbon-shaped and lack cross connections. The myonemes of the anterior cortex are flattened against the surface and are interconnected by an extensive system of cross branches. The inner surface of the frontal field is covered with a regularly cross-branched myoneme system which follows the curved pattern of frontal field kinety. The observed branching patterns and shapes of the myonemes support the hypothesis that they cause contraction of the cell. The membranellar root system was examined. Each membranellar root makes a 90° counterclockwise twist along its vertical axis (viewed from the inside) as it descends into the cell. The outer edge of each root fuses with the inner edge of the adjacent one, forming a continuous fiber sheet linking the roots together.     

Cortical ultrastructure of the Scuticociliates Dexiotricha media and Dexiotricha colpidiopsis (Hymenostomata).

Cortical ultrastructure of the scuticociliates Dexiotricha media and Dexiotricha colpidiopsis was investigated. The following elements of the somatic cortex were studied: the cell membrane, alveolar membranes and the epiplasm, kinetodesmal fibers, postciliary and transverse microtubular ribbons, and transverse fibers associated with single and paired kinetosomes; mitochondria and single microtubules located in interkinetal ridges; mature and early extrusion stages of mucocysts; the expulsion vacuole pore and tube, the nephridioplasm and the cytoproct. In the buccal cortex, the paroral kinety-ribbed wall complex, the 3 polykineties, and the cytostome-cytopharynx were investigated. Comparative survey of ciliate ultrastructure indicates 2 principal orientation patterns for kinetodesmal and postciliary fibers, recognition of which leads to reevaluation of the theory of paroral kinety formation and the ideas of homology based on this theory. Ultrastructurally, the scuticociliates are not distinct from tetrahymenines and peniculines; the 3 groups appear to be 1 assemblage.     

尼罗罗非鱼成熟卵结构及精子入卵早期的电镜观察

用扫描电镜观察尼罗罗非鱼(Tilopia nilotica)成熟卵卵膜孔结构和精子入卵的早期情况,用透射电镜观察成熟卵皮质,可见卵膜孔包括前庭和精孔管两部分,前庭壁及壳膜外表面上有许多小孔洞,精孔管壁呈阶梯状。卵膜孔下的卵皮质是一凹陷区,这一区域存在着皮质小泡。本实验见到5种形态的皮质小泡,其中大的皮质小泡靠近质膜。     

Cortical Ultrastructure of the Scuticociliates Dexiotricha media and Dexiotricha colpidiopsis (Hymenostomata)*†

SYNOPSIS. Cortical ultrastructure of the scuticociliates Dexiotricha media and Dexiotricha colpidiopsis was investigated. The following elements of the somatic cortex were studied: the cell membrane, alveolar membranes and the epiplasm, kinetodesmal fibers, postciliary and transverse microtubular ribbons, and transverse fibers associated with single and paired kinetosomes; mitochondria and single microtubules located in interkinetal ridges; mature and early extrusion stages of mucocysts: the expulsion vacuole pore and tube, the nephridioplasm and the cytoproct. In the buccal cortex, the paroral kinety-ribbed wall complex, the 3 polykineties, and the cytostome-cytopharynx were investigated. Comparative survey of ciliate ultrastructure indicates 2 principal orientation patterns for kinetodesmal and postciliary fibers, recognition of which leads to reevaluation of the theory of paroral kinety formation and the ideas of homology based on this theory. Ultrastructurally, the scuticociliates are not distinct from tetrahymenines and peniculines; the 3 groups appear to be 1 assemblage.     

EQUISETUM CLARNOI,A NEW SPECIES BASED ON PETRIFACTIONS FROM THE EOCENE OF OREGON

Equisetum clarnoi is described from four silicified stem fragments and numerous small roots from the Eocene Clarno Chert of Jefferson County, Oregon. Stems are up to 8.0 mm in diam and have sunken stomata arranged vertically in a single line flanking each of the external biangulate stem ridges, features that clearly ally this species with the subgenus Hippochaete. External stem ridges are equal in number to the carinal hypodermal bands. The hypodermis is composed of fibers and has prominent carinal bands up to 0.75 mm long and shorter vallecular bands. Cortical parenchyma cells enclose prominent vallecular canals which are lined by specialized thick-walled parenchyma cells. The double, common endodermis has prominent casparian strips. Vascular bundles are composed of four to seven metaxylem tracheids flanking each side of the phloem and protoxylem tracheids which occur singly on the internal surface of the small carinal canals. Leaf sheaths in cross section have an adaxial fibrous layer and an external or near external fibrous bundle. Roots are up to 2.0 mm in diam and have paired cuboidal epidermal cells from which root hairs arise. The stele of the root is central and shows exarch primary xylem maturation. Equisetum clarnoi most closely resembles the extant Equisetum hyemale var. affine.     

STUDIES ON THE STRUCTURE OF MUSCLE : III. PHASE CONTRAST AND ELECTRON MICROSCOPY OF DIPTERAN FLIGHT MUSCLE

1. The flight muscles of blowflies are easily dispersed in appropriate media to form suspensions of myofibrils which are highly suitable for phase contrast observation of the band changes associated with ATP-induced contraction. 2. Fresh myofibrils show a simple band pattern in which the A substance is uniformly distributed throughout the sarcomere, while the pattern characteristic of glycerinated material is identical with that generally regarded as typical of relaxed vertebrate myofibrils (A, I, H, Z, and M bands present). 3. Unrestrained myofibrils of both fresh and glycerinated muscle shorten by not more than about 20 per cent on exposure to ATP. In both cases the A substance migrates during contraction and accumulates in dense bands in the Z region, while material also accumulates in the M region. It is proposed that these dense contraction bands be designated the C_z, and C_m bands respectively. In restrained myofibrils, the I band does not disappear, but the C_z and C_m bands still appear in the presence of ATP. 4. The birefringence of the myofibrils decreases somewhat during contraction, but the shift of A substance does not result in an increase of birefringence in the C_z and C_m bands. It seems therefore that the A substance, if it is oriented parallel with the fibre axis in the relaxed myofibril, must exist in a coiled or folded configuration in the C hands of contracted myofibrils. 5. The fine structure of the flight muscle has been determined from electron microscopic examination of ultrathin sections. The myofibrils are of roughly hexagonal cross-section and consist of a regular single hexagonal array of compound myofilaments the cores of which extend continuously throughout all bands of the sarcomere in all states of contraction or relaxation so far investigated. 6. Each myofilament is joined laterally with its six nearest neighbours by thin filamentous bridges which repeat at regular intervals along the fibre axis and are present in the A, I, and Z, but not in the H or M bands. When stained with PTA, the myofilaments display a compound structure. In the A band, a lightly staining medullary region about 40 A in diameter is surrounded by a densely staining cortex, the over-all diameter of the myofilament being about 120 A. This thick cortex is absent in the I and H bands, but a thinner cortex is often visible. 7. It is suggested that the basic structure is a longitudinally continuous framework of F actin filaments, which are linked periodically by the lateral bridges (possibly tropomyosin). The A substance is free under certain conditions to migrate to the Z bands to form the C_z bands. The material forming the C_m bands possibly represents another component of the A substance. The results do not clearly indicate whether myosin is confined to the A bands or distributed throughout the sarcomere.     

Characteristics of the membranes of the pellicle of the ciliatePseudomicrothorax dubius

Summary The membranes of the pellicle of the ciliatePseudomicrothorax dubius are investigated using thin section electron microscopy and freeze-fracture replicas. The plasma membrane is covered by a surface coat and is connected to the outer alveolar membrane by short, sometimes branched, bridges. The inner alveolar membrane is coated on both sides. The epiplasm lies in intimate contact with the cytoplasmic surface of this membrane, and there is a corresponding deposit on the other surface. This deposit is regularly striated.The epiplasmic layer and the alveoli are interrupted at sites of cytotic activity,e.g., the attachment sites of trichocysts, the cytoproct, and the parasomal sacs. The striated deposit ends where the epiplasm ends, indicating a direct relationship between these two epimembranous layers.There is a deposit along the sides of the first part of the tip of the trichocysts, and in this region the trichocyst membrane is free of intramembranous particles.The membrane of the parasomal sacs has a coat on both surfaces. That on the extraplasmic surface is similar to the surface coat of the plasma membrane. The origin of the cytoplasmic coat is unknown. The cytotic activity of these sacs is indicated by their highly irregular profiles.     

A transient apical extracellular matrix relays cytoskeletal patterns to shape permanent acellular ridges on the surface of adult C. elegans

Epithelial cells secrete apical extracellular matrices to form protruding structures such as denticles, ridges, scales, or teeth. The mechanisms that shape these structures remain poorly understood. Here, we show how the actin cytoskeleton and a provisional matrix work together to sculpt acellular longitudinal alae ridges in the cuticle of adult C. elegans. Transient assembly of longitudinal actomyosin filaments in the underlying lateral epidermis accompanies deposition of the provisional matrix at the earliest stages of alae formation. Actin is required to pattern the provisional matrix into longitudinal bands that are initially offset from the pattern of longitudinal actin filaments. These bands appear ultrastructurally as alternating regions of adhesion and separation within laminated provisional matrix layers. The provisional matrix is required to establish these demarcated zones of adhesion and separation, which ultimately give rise to alae ridges and their intervening valleys, respectively. Provisional matrix proteins shape the alae ridges and valleys but are not present within the final structure. We propose a morphogenetic mechanism wherein cortical actin patterns are relayed to the laminated provisional matrix to set up distinct zones of matrix layer separation and accretion that shape a permanent and acellular matrix structure.     

Cellular events associated with lung branching morphogenesis including the deposition of collagen type IV

In this study mouse lung development was examined using an in vitro model system. The culture system permitted examination of a morphogenic process that eventually led to the formation of presumptive alveoli (terminal sacs). The observations included changes in epithelial cell morphology (transition from a columnar to a spindle shape), and evidence for motile activity on the part of primitive airway epithelial cells. The importance of Type IV collagen to the cellular events associated with branching morphogenesis was investigated by immunolocalization. In addition, we assessed the similarity of normal lung development to in vitro development by comparing cultured lungs with equivalent stages of embryonic and fetal mouse lungs. The results show that cultured embryonic lung explants proceed along a morphogenic pathway that parallels normal lung development; that primitive pulmonary epithelial cells engage in motile activity and transiently acquire an extended cell shape both in vitro and in vivo; that, as suggested by others, the pattern of late branching morphogenesis is not dichotomous, but irregular; and that short wisplike fibers of Type IV collagen are present in developing embryonic and fetal lung mesenchyme. Taken together, the results show that early and late lung branching patterns differ significantly, and suggest that later stages of lung branching involve distinct epithelial cell shape transitions. The immunofluorescence data suggest that fibrous Type IV collagen may be the extracellular matrix scaffold within which early epithelial cells accomplish lung branching morphogenesis.     

Morphogenesis of intestinal villi. I. Scanning electron microscopy of the duodenal epithelium of the developing chick embryo

Development of villi in the duodenum of the chick was studied in stages ranging from 11 days of incubation to one week after hatching. Formation of definitive villi is preceded by development of a set of previllous ridges that run lengthwise along the duodenum. The first set of 16 previllous ridges (Set I) is complete by about 13 days of incubation; all ridges in the set are fairly uniform and proceed through their subsequent development in synchrony. Previllous ridges in Set I fold into a highly regular zigzag pattern between 14 and 16 days of incubation. Definitive villi develop from Set I ridges beginning at about 17 days when populations of distinct cells appear on the crests of the ridges between angles in the zigzag folds. Cells in these populations lack the rounded appearance of cells seen in earlier stages; their apical surfaces are densely covered with microvilli. A second set of villi (Set II) develops at about 16 days of incubation when about 16 rows of tongue-like flaps erupt between the previllous ridges of Set I. At hatching, Set II villi are still smaller than villi of Set I; this distinction disappears by about the fourth day after hatching. The significance of the morphological changes in epithelial cells is discussed in terms of several hypotheses bearing on the mechanisms of villus formation.     

Scanning Electron Microscope Observations on Some Life History Stages of Carchesium polypinum (Ciliata,Peritrichia)1

SYNOPSIS. Sessile zooids, and mobile telotrochs and microgamonts of Carchesium polypinum (Protozoa, Ciliata, Peritrichia), were examined by scanning electron microscopy. The results were compared to earlier light and electron microscope studies in order to investigate structural changes concerned with adaptation and differentiation. Telotrochs and microgamonts always had a contracted peristome and usually had a long phalange of cilia. Striae around the contracted buccal apparatus in all 3 stages were convoluted and often had thickened margins; those in telotrochs and microgamonts had oral-aboral ectoplasmic cross-connections. Nonbuccal striae of telotrochs and microgamonts varied in structure and height differences between epiplasmic peaks and alveoli surface membranes. The number of striae were constant in all 3 stages. Pellicular pore structure did not vary in any of the stages examined and resembled parasomal sacs located near buccal structures. Fully relaxed sessile zooids had ectoplasmic ridges coursing from polykinety kinetosomes and cilia to an area in front of the ciliated portion of the haplokinety; these ridges were interpreted to be the interkinetal fibers. Telotroch bands of sessile zooids consisted of 2 or 3 parallel ectoplasmic ridges which circled the aboral region and contained structures resembling pores. Telotroch bands in telotrochs and microgamonts had 2 enlarged, parallel ectoplasmic ridges circling the aboral region; telotroch band cilia were found between these ridges. In addition, a fold-like, ectoplasmic structure extended beyond the 2 ridges and was located between the telotroch band cilia and the aboral ridge. The epiplasmic shelf surrounding the stalk in sessile zooids was enlarged in telotrochs, and cilia were seen in the scopula depression. No scopula organelle was seen in any microgamont.     

Internalization of macromolecules from the medium in Suctoria

Takophrya infusionum like all other Suctoria lacks an oral cavity. Its feeding apparatus consists of tentacles, long narrow tubes through which the contents of the living prey are ingested. For normal growth, reproduction, and longevity of clones, Tokophrya needs supplements deriving from the medium in addition to living prey. Since Tokophrya lacks a mouth, these supplements can reach the cytoplasm only through the complex structure of the cortex, which is composed of a three- membraned pellicle and a dense epiplasm. In addition, external to the cortex, an extraneous coat covers the whole organism. Only the outer pellicular plasma membrane is continuous; the other two and the epiplasm are interrupted by the outer plasma membrane which invaginates at intervals forming the so-called pits. The invaginated plasma membrane dips down into the cytoplasm where it extends to form a saccule. Experiments with cationized ferritin and Thorotrast provide evidence that internalization of these macromolecules takes place through the pits by pinocytosis. The membrane of the saccules of the pits forms invaginations which pinch off giving rise to small, flattened vesicles containing the tracers. The tracers were never found free in the cytoplasm but exclusively in the flat vesicles. These vesicles are thus the vehicles transporting macromolecules from the medium to the cytoplasm. The saccules of the pits are the natural loci of pinocytosis and together with the flattened vesicles perform an important function in Suctoria, supplying the organisms with macromolecules from the medium.     

Ciliary Bands in Echinoderm Larvae: Evidence for Structural Homologies and a Common Plan

Abstract A series of laterally projecting ridges develop along the ciliary band of late stage auricularia larvae. These are similar in position to the larval arms of bipinnaria larvae and the epaulettes and vibratile lobes of echinoid pluteus larvae, all of which structures are potentially homologous. When the auricularia is converted to a doliolaria with a series of circumferential ciliary bands, the ridges of the former are retained as basic elements from which the circumferential bands of the latter then develop. There is a simple repeating pattern in the arrangement of these elements in which bands composed of two elements alternate with bands composed of four. The available evidence does not resolve the question of which of the above four larval types, whether feeding or non-feeding, is more primitive. The common plan apparent among them suggests, however, that this plan, whatever its origin, predates the diversification of larval types among eleutherozoan echinoderms.