Spermiogenesis in the Oligochaetoid Polychaete Questa (Annelida, Questidae)

The spermatids are connected to a central cytophore by cytoplasmic bridges and are polarized in the sequence: empty cytoplasm; uncondensed nucleus; mitochondria which surround the distal region of the nucleus and the centrioles; axoneme; posterolateral to the base of the axoneme, the Golgi apparatus and (when secreted) the acrosomal rudiment. The dome-shaped acrosome vesicle elongates progressively as it migrates to the tip of the elongating and condensing nucleus; subacrosomal material gives rise to an almost equally long, tubular, thick-walled perforatorium. After the acrosome has greatly elongated, the mitochondria are reduced to two, which lose their rounded form and invest the growing axoneme to give a very elongate midpiece. Transfer of materials from nucleus to mitochondria is discussed. Microtubules surrounding the acrosome and nucleus disappear by maturity, but those internal to the mitochrondria apparently persist as the accessory microtubules, unique in the Annelida, which surround the 9 + 2 axoneme. Microvilli of the egg envelope, which have tetrads of terminal branches (epivitelline projections) resembling epicuticular projections, are less than 1 μm long, whereas the mature acrosome exceeds 5 μm. This suggests that the correlation seen in oligochaetes is absent.     

Ultrastructure of the metanephridial system in Aeolosoma bengalense (Annelida)

Summary The excretory system of Aeolosoma bengalense has been examined by light and electron microscopy. The system consists of seven serially arranged paris of metanephridia and six pairs of podocytes (referring to the first zoid of an animal chain). The podocytes surround blood spaces of the alimentary canal forming dorsoventrally running loops that emerge on both sides of it. The two elements of the system have a correlative position, each podocyte extending in close proximity to the funnel of a metanephridium. Only in the region of the first metanephridia are podocytes lacking. The nephrostome of the metanephridia consists of two cells, an inner one, the terminal duct cell, and an outer one enwrapping it, called the mantle cell. Nephrostomal cilia that extend into the coelomic space arise exclusively from the rim of the mantle cell whereas those of the terminal duct cell arranged on its luminal surface protrude into the canal forming a flame. The nephridial canal is ciliated throughout and is either intra- or extracellular. Its initial loops aggregate to form a compact organ, the nephridial body. The middle part of the duct constitutes a loop that ascends at each side of the alimentary canal where it is in intimate contact with its blood spaces. Ultrastructural features of the duct cells suggest a reabsorptive function in two regions, the nephridial body and the uppermost part of the loop. The terminal part of the duct passes through the nephridial body and opens ventrolaterally. Generally, the transverse vascular loops at the gut consist of one podocyte each. In the oesophageal region, where only one pair of podocytes is present, the loops connect the dorsal with the ventral longitudinal vessel. Three pairs of podocytes are present in the dilated region of the intestine emerging from its lateral wall and joining the median ventral vessel or blood spaces near by. In the hind gut, where two pairs of podocytes occur, the loops arise from the dorsolateral part and enter directly the ventral vessel. Cytological features of podocytes resemble those of other animals. The results are discussed on the basis of current theories on the function and the phylogenetic significance of excretory systems in the Annelida.Abbreviations bl basal lamina - bs blood space - bv blood vessel - cf ciliary flame - ci cilia - co connection of the vascular loop with the intestinal blood space - cu cuticle - db dense body - dc duct cell - di dictyosome - za zonula adhearens - dv dorsal vessel - ecb epicuticular body - ev endocytotic vesicle - ic intestinal cell - ici inner cilia - iv intestinal vessel - lm longitudinal muscle - mc mantle cell - mg midgut - mi mitochondrion - mv microvilli - nu nucleus - oci outer cilia - oe oesophagus - pc podocyte - pe pedicel - pel primary elongation of the podocyte - sm slit membrane - tc terminal duct cell - ve vesicle with heterogeneous contents - vv ventral vessel     

Ultrastructure of the Tubificid Acrosome (Annelida, Oligochaeta)

The later morphogenesis of the acrosome of Limnodriloides winckelmanni and Rhyacodrilus arthingtonae is compared with that in Enchytraeus and in earthworms. After superposition of the acrosome on the tip of the nucleus the manchette continues apically beyond the nucleus to ensheath the acrosomal tube. At the posterior limit of, and probably contained in, the spacious/ terminal primary acrosomal vesicle is an electron-dense ring. A domed protrusion into the floor of the primary vesicle is tentatively regarded as the secondary acrosome vesicle. The axial rod when first observed is attached to the vesicle complex. Later, the rod detaches and extends deeply into the acrosome tube. A membrane ensheathes the tubificid axial rod but its exact homology with the complex layers surrounding the lumbricid or megascolecid axial rod is not clear. The domed apical region of the tubificid acrosome is probably a persistence of the primary acrosome vesicle and it is deduced that the acrosome vesicle surrounding the axial rod in lumbricids and megascolecids is a product, by invagination, of the secondary acrosome vesicle only.     

Ultrastructure of the nephrostome in Enchytraeus albidus (Annelida, Clitellata)

 The nephrostome of Enchytraeus albidus exhibits a longitudinal slit-like opening on the dorsal side of a bulbous organ which is mainly composed of three cells, one flame cell situated centrally and endowed with a ciliary flame, and two cells lying superficially, called the mantle cell and the accessory mantle cell. The mantle cell occupies the ventral side of the organ extending on both sides up to the opening to constitute its immediate border on the frontal and lateral sides. Here it forms a kind of crest which is heavily subdivided into many protrusions and infoldings endowed with long cilia which exclusively spread into the coelomic cavity. The accessory mantle cell borders the narrow posterior slit of the opening, forming the roof of the initial canal which is devoid of cilia. From its anterior region a projection arises extending above the opening. The flame cell forms a groove from which the ciliary flame arises which extends into the canal. At its posterior region it replaces the accessory mantle cell displacing it onto the dorsal surface of the organ. It is argued that the mantle cell and the accessory mantle cell have presumbly originated from coelothelial cells. Thus the metanephridium may be a composite organ developing from a nephridioblastic and a coeloblastic source. A discussion of results in other annelid species indicates that metanephridia in the Annelida may have evolved more than once. Accepted: 13 October 1997     

Ultrastructure of the eyespot in three polychaete trochophore larvae (Annelida)

Summary The eyespot is structurally similar in trochophore larvae of Harmothoe imbricata, Serpula vermicularis and Spirobranchus giganteus. In the receptor cell parallel lamellae lie below a tuft of microvilli which extends into a hollow in one side of the pigment cell. In 1-eyed trochophores this space connects with the outside via a small pore. In eyes preserved during the day there is evidence of a membrane breakdown in both lamellae and microvilli as well as indications of a membrane-fragment disposal system involving the receptor cell, the pigment cell and the eyespot pore. The orientation of the eyespot of S. giganteus is the reverse of that found in S. vermicularis, a situation that may be associated with ecologically significant differences in behaviour.     

Ultrastructure of Chrysopetalid Paleal Chaetae (Annelida,Polychaeta)

Paleal notochaetae belonging to a number of Chrysopetalum species (Chrysopetalidae) were examined by scanning and transmission electron microscopy. Paleae are composed of broad medullary channels stacked with a regular series of horizontal fibrous diaphragms. The medullary part of the palea is surrounded by irregular rows of narrow tubular channels within the chaetal cortex. The origin and function of camerate chaetae is discussed.     

Ultrastructure and development of the nephridia in Anaitides mucosa (Annelida,Polychaeta)

Different developmental stages (trochophores, nectochaetae, non-mature and mature adults) of Anaitides mucosa were investigated ultrastructurally. A. mucosa has protonephridia throughout its life; during maturity a ciliated funnel is attached to these organs. The protonephridial duct cells are multiciliated, while the terminal cells are monociliated. The single cilium is surrounded by 14 microvilli which extend into the duct lumen without coming into any contact with the duct cells. Corresponding ultrastructure and development indicate that larval and adult protonephridia are identical in A. mucosa. Differences between various developmental stages can be observed only in the number of cells per protonephridium. A comparison between the funnel cells, the cells of the coelothel and the duct cells reveals that the ciliated funnel is a derivative of the duct. Due to the identical nature of the larval and postlarval protonephridia, such a funnel cannot be a secondary structure. In comparison with the mesodermally derived metanephridial funnel in phoronids it seems likely that the metanephridia of annelids and phoronids evolved convergently.     

Ultrastructure and significance of the transitory nephridia in Erpobdella octoculata (Hirudinea, Annelida)

In early developmental stages of Erpobdella octoculata two pairs of transitory nephridia occur which degenerate during the formation of the body segments. Because in the ground pattern of Annelida the first nephridia formed during ontogenesis are protonephridia, it can be assumed that the transitory nephridia of E. octoculata are homologous to the larval protonephridia (head kidneys) of Polychaeta. To test this hypothesis two cryptolarvae of E. octoculata were investigated ultrastructurally. Both pairs of transitory nephridia are serially arranged to either side of the midgut vestigium. Each organ consists of a coiled duct that opens separately to the exterior by an intraepidermal nephridiopore cell. The duct is percellular and formed by seventeen cells. Adluminal adherens and septate junctions connect all duct cells; the most proximal duct cell completely encloses the terminal end of the duct lumen. A filtration structure characteristic for protonephridia is lacking. Additionally, the entire organ lacks an inner ciliation. Morphologically and ultrastructurally the transitory nephridia of E. octoculata show far reaching congruencies with the segmental metanephridia in different species of the Hirudinea. These congruencies support the assumption that formation of transitory nephridia and definitive metanephridia in Hirudinea depends on the same genetic information. The same inherited information is assumed to cause the development of larval head kidneys and subsequently formed nephridia in different species of the Polychaeta. Thus, the presumed identical fate of a segmentally repeated nephridial anlage supports the hypothesis of a homology between the transitory nephridia in Hirudinea species and the protonephridial head kidneys in the ground pattern of the Polychaeta. We, therefore, assume that functional constraints lead to a modification of the protonephridial head kidneys in Hirudinea and explain ultrastructural differences between the transitory nephridia in Hirudinea and the protonephridia in Polychaeta. Accepted: 11 December 2000     

Ultrastructure of the Genital Organs in the Interstitial Syllid Petitia amphophthalma (Annelida,Polychaeta)

Abstract The gonochoristic syllid Petitia amphophthalma is one of the truly interstitial polychaetes. P. amphophthalma does not show any epitokous modifications at maturity such as those that usually occur in syllids. The reproductive structures are unique: the male genital organs consist of a seminal vesicle in chaetigers 6–10, subdivided into a dorsal part tightly filled with spermatozoa and a ventral part with contents in different stages of spermatogenesis, one pair of sperm ducts and conspicuous gland cells situated in chaetigers 10 and 11. Their glandular secretions are discharged into the sperm duct together with those of other types of gland cells that form the duct. The oocytes develop freely within the body cavity of the females. Each of the fertile segments possesses a paired oviduct ending in a large ciliated funnel. Sperm ducts and oviducts are probably modifications of excretory organs; nephridia are absent in segments where gonoducts occur. A direct sperm transfer by lytic opening of the integument of the female and internal fertilization are inferred. Copyright © 1996 Published by Elsevier Science Ltd on behalf of the Royal Swedish Academy of Sciences     

Ultrastructure of gill epithelial cells of Diopatra neapolitana (Annelida,Polychaeta)

Summary The ultrastructure of gill epidermal cells of Diopatra neapolitana and their relationship with blood spaces are described. The existence of a basal infolding complex, related to the blood spaces, is also reported. A possible involvement of these cells in osmoregulation and ion interchange, apart from their well-known role in respiration, is suggested.Abbreviations bc Blood cell - bi Basal infolding - bl Basal lamina - bs Blood space - ci Cilia - cu Cuticle - db Dense body - EC Epidermal cell - Gc Golgi complex - id Interdigitation - j Junction - m Mitochondria - mv Microvilli - n Nucleus - pv Pinocytotic vesicle - rer Rough endoplasmic reticulum     

Ultrastructure and functional morphology of the female reproductive organs inProtodrilus (Polychaeta,Annelida)

The morphology and function of the female reproductive organs in 6Protodrilus species are investigated by light- and transmission electron microscopy. Possible ways in which spermatozoa may enter the female coelom after leaving the spermatophore are discussed for species with and without special female reception organs. Only femaleP. rubropharyngeus andP. flavocapitatus have “dorsal organs” for spermatophore reception. The structure and function of these organs are described, as well as those of the oviduct found in 3 of the species investigated. The possible phylogenetic origin of gonoducts and different modes of oviposition within the genus are discussed. Finally, the high taxonomic significance of female traits such as dorsal organs, oviducts, cocoon glands and lateral ciliary rows in this genus is stressed.     

Ultrastructure of the male nephridium and its role in spermatophore formation in spionid polychaetes (Annelida)

Summary The mature male nephridia ofPolydora ligni andP. websteri (Polychaeta: Spionidae) are segmental organs composed of a ciliated nephrostome connected to a nephridial canal that crosses the intersegmental septum, expands into a large modified part extending dorsally through the coelom and subsequently narrows into a canal terminating in a dorsal nephridiopore. The nephridial canal is ciliated throughout and is composed of several cell types. Cells in the expanded region of the nephridia of both species contain large urn-shaped depressions filled with long microvilli. InP. ligni, one section of a nephridium contains cells packed with electron-dense granules that are not observed inP. websteri.The spermatophores ofPolydora ligni are composed of a central sperm mass surrounded by a layer of randomly oriented tubules that form a capsule around the sperm and taper into a long thin tail. These tubules are identical in dimensions to the microvilli present in parts of a nephridium and apparently are derived from these microvilli. The spermatophore capsule ofP. websteri is composed of similar tubules also presumed to originate from nephridial microvilli.The microvilli in nephridia of both species are surrounded with a glycocalyx that may function as an adhesive to hold the spermatophore capsule together. This glycocalyx may also function as a species specific message when encountered by a receptive female.Contribution Number 179 from Harbor Branch Foundation, Inc.     

Spermatozoal Ultrastructure: Evolution and Congruence with a Holomorphological Phylogeny of the Oligochaeta (Annelida)

Plesiomorph characters for the oligochaete spermatozoon are proposed. The chief trends from these plesiomorphies have been elongation of the acrosome and its tube; withdrawal of the primary acrosome vesicle and the axial rod into the acrosome tube and development of a capitulum; development of connectives from the secondary tube to the axial rod (though there is some possibility that the reverse, absence of connectives, is plesiomorph); detorting and shortening of the midpiece (or possibly, again, the reverse) with an increase in numbers of mitochrondria from the plesiomorph four to eight; modification of the base of the tube to form a hen of variable form; and, in one line (lumbricids) flattening of the tip of the nucleus and correspondingly of the limen. Sperm ultrastructure, examined for 9 oligochaete families, corresponds well with taxonomic and phylogenetic groupings recently recognized by the author. However, convergent similarity of the phreodrilid sperm to that of the Lumbricina suggests a corresponding alteration of fertilization biology in the phreodrilids. The results indicate that the Haplotaxidae lie at the base of the opisthopores, though they do not unequivocally contraindicate acceptance of a Haplotaxis-like form as a stem form of the Haplotaxida (opisthopores and Haplotaxidae) and Tubificida. An even more basal position for prosopores, now represented by the Lumbriculida, cannot yet be dismissed.     

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells. One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny.     

Ultrastructure of the preseptal part of metanephridia in Nais variabilis and Dero digitata (Annelida, Clitellata)

The composition and arrangement of cells in the preseptal region of metanephridia have been examined by ultrastructural methods in two naidid species, Nais variabilis and Dero digitata. Within this region special attention has been paid to the portion around the orifice and the region where the metanephridium penetrates the septum. In N. variabilis, the preseptal region is composed of four cells and, in D. digitata, three cells are present. In both species three cells correspond in position and ultrastructural details and, hence, are interpreted as homologous. These are the mantle cell, the flame cell, and the canal cell. The mantle cell covers the preseptal region and surrounds the opening. The margin around the orifice is endowed with cilia, which extend into the coelomic space and beat irregularly. They do not enter the orifice and, thus, are not part of the internal ciliary flame. Posteriorly, in D. digitata, the mantle cell originates from the septal wall, i.e., its extensions spread in the plane of the frontal coelothelium of the septum. In N. variabilis, the mantle cell is continued by a further cell, enwrapping the posterior region of the preseptal part. This cell, called the septal cell, is anchored in the septal wall like the mantle cell in D. digitata. Both cells are interpreted as mesodermal components of the metanephridium. The flame cell lies beneath the mantle cell. In front, on its dorsal wall, many cilia are inserted which extend posteriorly into the nephridial canal forming a flame. In D. digitata, the caudal extension of this cell was examined in more detail; it originates from an intraseptal position. The canal cell lines the anterior lumen of the nephridial duct. While the mantle cell and flame cell enclose the organ from a dorsal position, the canal cell lies opposite embracing the lumen from a ventromedial position. Behind, it extends into the postseptal region for a certain distance. It is concluded that metanephridia in the Clitellata have a coelothelial component and, probably, are not just descendants of a single cell, the nephridioblast. The results further indicate that a flame cell and a mantle cell or some corresponding coelothelial cells may be constitutive elements of the ground plan of the clitellate metanephridium. Phylogenetic consequences for non-clitellate Annelida are discussed. Accepted: 21 December 1999     

Ultrastructure of the body wall of three species of Grania (Annelida: Clitellata: Enchytraeidae)

De Wit P., Erséus C. and Gustavsson L.M. 2011. Ultrastructure of the body wall of three species of Grania (Annelida: Clitellata: Enchytraeidae). —Acta Zoologica (Stockholm) 92 : 1–11. The body wall of three species of Grania, including the cuticle, epidermis and the musculature, are studied using TEM. The cuticle is similar to previously studied enchytraeids, with an orthogonal grid pattern of collagen fibers. This pattern is also seen in Crassiclitellata, which has been suggested as the sister taxon of Enchytraeidae. Variation of epicuticular and fiber zone patterns seen in Naididae (formerly Tubificidae and Naididae) seem to be lacking in Enchytraeidae. The fiber thickness, however, varies between Grania species and may be a phylogenetically informative character. The epidermis consists of supporting cells, secretory cells and sensory cells. Basal cells, typical for Crassiclitellata, were not observed. The clitellum of Grania seems to consist of two types of gland cells, which develop from regular epidermal tissue. It is possible that more cell types exist in different regions of the clitellum, however. The body wall musculature is arranged somewhat differently from that of closely related taxa; this refers to the reduction of circular and outer, triangular longitudinal muscle fibers, while the inner, ribbon‐shaped longitudinal muscle fibers are well‐developed. A search was conducted for the cause of the peculiar green coloration of Grania galbina De Wit and Erséus 2007, and it was concluded that neither cyanobacteria nor epidermal pigment granules were present in the fixed material.     

Ultrastructure of the extensively developed nuchal organs of Laonice bahusiensis (Annelida: Canalipalpata: Spionidae)

The nuchal organs of annelid Laonice bahusiensis (Spionidae) from northern Europe have been studied using scanning and transmission electron microscopy. L. bahusiensis is the first spionid species in which extensively developed, continuous nuchal organs are described. The nuchal organs of this genus are the longest known among polychaete annelids. They consist of paired double bands extending from the prostomium on a mid‐dorsal caruncle for about 24–30 setigers. Their microanatomy corresponds to the general structural plan of nuchal organs: there are ciliated supporting cells and bipolar sensory cells with sensory cilia traversing an olfactory chamber. The organs are overlaid by a secondary paving‐stone‐like cover and innervated by one pair of longitudinally elongated nuchal nerves. These findings clearly favor the hypothesis that the paired, extensively developed ciliated structures found in some Spionidae are homologous with the prostomial nuchal organs characteristic of polychaete annelids. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Ultrastructure of the anterior salivary gland cells of the giant leech, Haementeria ghilianii (Annelida, Hirudinea)

The giant anterior salivary gland cells from the large mammalian blood-sucking, glossiphoniid leech, Haementeria ghilianii, can be subdivided into three morphologically and functionally distinct regions: 1) a soma, responsible for the synthesis and storage of secretory products; 2) a long cell process, responsible for the storage and intracellular transport of the secretory vesicles; and 3) the site of exocytosis at the process terminal. The giant somata are densely packed with secretory vesicles. Deep plasmalemmal invaginations invade the soma and form an extensive system of extracellular lacunae. The rough endoplasmic reticulum (ER) and the Golgi apparatus are organized in the cell periphery, near the highly branched nucleus, and along the lacunae. The somata taper into long processes extending over several centimeters to the proboscis tip. These contain secretory vesicles through their whole length. In the process periphery, the vesicles are completely ensheathed by a concentric subplasmalemmal smooth ER cisterna. This originates deeply within the soma and extends through the whole cell process to its terminal. The ER provides support for up to several hundred longitudinally oriented microtubules. Secretion occurs at the very tip of the cell processes, each of which terminates at the proboscis tip at the base of a cuticular pore. We found synapses close to the sites of exocytosis, providing morphological evidence for neuronal control of secretion.     

The Protonephridia of the Arctic Kinorhynch Echinoderes aquilonius (Cyclorhagida,Echinoderidae)

The cyclorhagid kinorhynch Echinoderes aquilonius Higgins & Kristensen, 1988 possesses a single pair of protonephridia located in segments 10 and 11. The protonephridia consist of: (1) three terminal cells T-1, T-2. T-3, each with two cilia; (2) a single non-ciliated canal cell; (3) a nephridiopore cell with many microvilli and a cuticular sieve plate. The protonephridia of Echinoderes are presumed to develop from the ectoderm near the area of the sieve plate on the eleventh segment, and are suspended in the dorso-lateral pseudocoelomic cavity where they are surrounded by a basal lamina. One of the terminal cells (T-1) secondarily penetrates the basal lamina of the tenth segment and a part of the cell attaches to the cuticle. The kinorhynch protonephridia are compared with the excretory organs of other Bilateria. expecially the ‘aschelminths’, and apomorphic characters of the kinorhynch protonephridia are defined.