Comparative electron microscopic study of the stomach of Orchestia cavimana and Arcitalitrus sylvaticus (crustacea: Amphipoda)

The functional morphology of stomachs of the European semiterrestrial amphipod Orchestia cavimana and of the Australian terrestrial species Arcitalitrus sylvaticus was studied by electron microscopy. The stomach of the two amphipod species is divided longitudinally into a spacious dorsal food channel and two ventral filtration channels. Additionally, a prominent helically oriented circulation channel is situated on each lateral side of the stomach, forming a semicircular channel separated from the food channel by spines. The food channel conveys coarse food particles directly into the midgut through a funnel. The filtration channels receive fine material filtered through primary and secondary filters. Material forced through the secondary filters by the pressure of the laterally located inferolateralia eventually reaches the openings of the midgut glands. Washing of filters and soaking of ingested food items with enzymes probably is achieved by a forward stream of digestive juice from the midgut glands and conveyed through the circulatory channels. The specializations of the stomach of the two species of Amphipoda investigated are described and compared to the pertinent structures of Mysidacea and Isopoda.     

Microscopic anatomy,functional morphology,and ultrastructure of the stomach of Euphausia superba Dana (Crustacea,Euphausiacea)

Summary Microscopic anatomy, functional morphology, and ultrastructure of the stomach of the antarctic krill Euphausia superba Dana were investigated by means of serial sections, scanning and transmission electron microscopy, and video technique. A separation of the stomach into an anterior part, called cardia, and a posterior part, called pylorus, became evident. Protrusions of the stomach into the midgut form the third region, called the funnel. The interior of the cardia is dominated by the two lateralia, originating from the side walls of the stomach. At their undersurface, they bear the primary filter. It separates the dorsal food channel from the ventral filtration channel, which is divided into two channels by a ventromedian ridge, the anteromedianum. Within the pylorus, the inferolateralia act in sealing the food channel from the filtration channel. In contrast to many other Malacostraca, the inferomedianum bears no secondary filter. During live observations, the stomach of Euphausia superba shows distinct pumping phases. A comparison of the structure of the stomach with data obtained from other Crustacea will lead to a better understanding about the relationships between the Malacostraca.     

Functional morphology of the stomach of Corophium volutator Pallas

Corophium volutator Pallas is a small amphipod crustacean which burrows in inter-tidal mud on the British coast, and feeds on organic detritus, mainly vegetable, by selecting particles from the mud.
The alimentary canal consists of foregut, midgut and hindgut. The midgut is produced into a pair of anterior dorsal caeca, a pair of ventral caeca and a pair of posterior dorsal caeca.
The cardiac stomach has a large number of chitinous plates or ridges beset with hooks and spines for the trituration of food.
The pyloric stomach has long fine bristles which form an eifective filter apparatus and allow only fine particles of food to pass into the midgut, where they are digested and absorbed.
The hepato-pancreas secrete digestive enzymes and store reserve food material as oil globules.     

Microscopic anatomy and ultrastructure of the stomach of Porcellio scaber (Crustacea,Isopoda)

Summary From the micromorphological viewpoint, the stomach is by far the most complicated part of the digestive tract of Porcellio scaber. All parts are shown in electron micrographs. The simplest inner surface of the stomach is the dorsal part, which has an unpaired flap directed posteriorly (lamella dorsalis). The lateral walls carry the lateralia, superolateralia and lamellae annulares. The most complicated region of the stomach is the ventral surface with its anterior primary filter, the adjacent masticatory areas, the posterior secondary filter on the lateral sides of the inferomedianum and the inferolateralia. Ingested food can be filtered twice: first on the primary filter and then on the secondary filter. The digestive fluid secreted in the midgut glands can be conveyed on the same route as the filtrate of the gastric juice, but in the other direction. The posterior part of the stomach is wrapped by a ring-shaped fold of the anterior hindgut, thus making seven epithelia closely apposed to each other.     

The filter pads and filtration mechanisms of the devil rays: Variation at macro and microscopic scales

Three lineages of cartilaginous fishes have independently evolved filter feeding (Lamniformes: Megachasma and Cetorhinus, Orectolobiformes: Rhincodon, and Mobulidae: Manta and Mobula); and the structure of the branchial filters is different in each group. The filter in Rhincodon typus has been described; species within the Lamniformes have simple filamentous filters, but the anatomy and ultrastructure of the branchial filter in the mobulid rays varies and is of functional interest. In most fishes, branchial gill rakers are elongated structures located along the anterior ceratobranchial and/or epibranchial arches; however, mobulid gill rakers are highly modified, flattened, lobe‐like structures located on the anterior and posterior epibranchial elements as well as the ceratobranchials. The ultrastructure of the filter lobes can be smooth or covered by a layer of microcilia, and some are denticulated along the dorsal and ventral lobe surface. Flow through the mobulid oropharyngeal cavity differs from other filter‐feeding fishes in that water must rapidly deviate from the free stream direction. There is an abrupt 90° turn from the initial inflowing path to move through the laterally directed branchial filter pores, over the gill tissue, and out the ventrally located gill slits. The deviation in the flow must result in tangential shearing stress across the filter surface. This implies that mobulids can use cross‐flow filtration in which this shearing force serves as a mechanism to resuspend food particles initially caught by sieving or another capture mode. These particles will be transported by the cross filter flow toward the esophagus. We propose that species with cilia on the rakers augment the shear mediated movement of particles along the filter with ciliary transport. J. Morphol. 274:1026–1043, 2013. © 2013 Wiley Periodicals, Inc.     

Pharynx and intestine

The alimentary canal of polychaetes consists of a foregut, midgut, and hindgut. The alimentary canal shows different specializations even in homonomously segmented polychaetes. The foregut gives rise to the buccal cavity, pharnyx and oesophagus, the midgut may be divided into a stomach and the intestine proper. Since polychaetes use a wide spectrum of food sources, structures involved in feeding vary as well and show numerous specializations. In the foregut these specializations may be classified as one of the following types: dorsolateral folds, ventral pharynx, axial muscular pharynx, axial non-muscular proboscis and dorsal pharynx. The latter, typical of oligochaetous Clitellata, occurs rarely in polychaetes. The structure, evolution and phylogenetic importance of these different types are described and discussed. Axial muscular and ventral pharynges may be armed with jaws, sclerotized parts of the pharyngeal cuticle. Terminology, structure, occurrence and development of the jaws are briefly reviewed. Special attention has been paid to the jaws of Eunicida including extinct and extant forms. Conflicting theories about the evolution of the jaws in Eunicida are discussed. The epithelia of the intestine may form a pseudostratified epithelium composed of glandular cells, absorptive cells and ciliated cells or only one cell type having similar functions. A conspicuous feature in the intestine of certain polychaetes is the occurrence of unicellular tubular structures, called enteronephridia. So far these enteronephridia are only known in a few meiofauna species.     

Observations on nutrition and ultrastructure of digestive tract and fat body of the giant paranthurid Accalathura gigantissima Kussakin

Summary The giant Antarctic paranthurid Accalathura gigantissima Kussakin (Crustacea, Isopoda) is adapted towards the predation of amphipods, which are pierced and sucked out by means of specialized mouthparts. Oesophagus and stomach pump the food first into the highly dilatable hindgut, later the chyme is transported into the digestive glands, whereby larger particles are retained in the intestine by the filters of the stomach. The ultrastructure of the hindgut shows the features of a transporting epithelium, no indications for secretion of digestive enzymes, intracellular digestion or storage of lipids or glycogen could be found. The digestive glands have very active cells which are responsible for resorption and secretion, intracellular digestion and storage of lipids (probably triglycerides). Lipids are metabolized and transported without the presence of SER, which is replaced by RER. Adipose tissue is present between muscles and forms laterally and ventrally a hypodermal fat body that can be rather voluminous in female specimens. The adipocytes of the studied immature females contained besides lipids and glycogen a large number of electron-dense granula probably composed of lipoproteins, which might be precursors of yolk material.     

Surface ultrastructure of gill arches and gill rakers in relation to feeding of an Indian major carp, Cirrhinus mrigala

The surface ultrastructure of the gill arches and the gill rakers of an herbivorous fish, Cirrhinus mrigala was investigated by scanning electron microscopy. These structures show significant adaptive modifications associated with the food and feeding ecology of the fish. Closely lying short gill rakers and narrow inter-raker channels on the gill arches are associated to filter and retain food particles. Prominent epithelial protuberances on the gill rakers and the gill arches enable the taste buds, located at their summit, to project well above the surface of the epithelium. This could increase the efficiency of the taste buds in selective sorting of palatable food. Surface specializations of the postlingual organ are recognized adaptive modifications for selecting, trapping or holding food particles. Prominent molariform teeth born on the lower pharyngeal jaw, and the chewing pad opposite it, are associated to work together as an efficient pharyngeal mill. Mucous goblet cells are considered to elaborate mucus secretions to trap, glue and lubricate food particles for their smooth transport for swallowing.     

The anatomy and histology of the midgut and hindgut of Charybdis (Goniohellenus) truncata (Fabricius, 1798) (Decapoda: Brachyura)

The midgut of C. (G.) truncata accounts for half of the postgastric intestinal tract. The paired anterior midgut caeca arise just behind the pyloric stomach, on either side of the midgut. The unpaired posterior midgut caecum arises dorsally at the rear end of the midgut, where this joins the hindgut. The midgut and its caeca help in the digestive absorption of food. The hindgut is of ectodermal origin and is lined with chitin of a collagenous nature. The connective tissue of the anterior part of the hindgut is packed with tegumental glands whose secretion contains both sulphated and weakly acidic mucosubstances, which facilitate the passage of faecal matter and help to bind food particles. The digestive gland - the hepatopancreas - opens into the anterior part of the midgut, below the anterior midgut caeca. Histologically, its tubules contain three different types of cells - "F", "R" and "B" cells.     

Studies on water and ion transport in homopteran insects: ultrastructure and cytochemistry of the cicadoid and cercopoid midgut

The midgut of cicadoid and cercopoid insects is differentiated at the anatomical, ultrastructural and cytochemical levels into a conical segment, anterior, mid, and posterior midgut. The cells of the conical segment and anterior midgut are cytochemically very similar. They differ in ultrastructure, the anterior midgut cells having a submicrovillar row of mitochondria and a very marked mucoprotein coat investing the microvilli. The mid-midgut contains mineral spherites, which are formed in cisternae in the endoplasmic reticulum, and ferritin. The posterior midgut differs cytochemically from the anterior midgut and the cells are characterized by deep narrow basal invaginations and the absence of a mucoprotein coat investing the microvilli. It is suggested that nutrient absorption occurs in the conical segment and anterior midgut. Ion absorption may also occur in the anterior midgut. Storage excretion of calcium, magnesium and phosphate occurs in the mid-midgut. Ferritin is also stored here but may be found in other regions of the midgut, particularly in the cicada. The posterior midgut may be involved in ion secretion which could be related to filter chamber function.     

Studies on water and ion transport in homopteran insects: ultrastructure and cytochemistry of the cicadoid and cercopoid Malpighian tubules and filter chamber

The filter chamber is a complex junction of anterior and posterior extremities of the midgut and Malpighian tubules. The sac-like anterior extremity, or filter chamber proper, comprises two cell types. These are large cuboidal cells which secrete a mucoprotein, and extremely thin cells which have regular tubular invaginations of the basal plasma membrane. The posterior extremity of the midgut and the internal Malpighian tubules coil round the filter chamber proper. They consist of thin epithelial cells identical in ultrastructure. The basal plasma membrane in these cells is formed into leaflets. A thin cellular sheath and thick muscle layers surround the filter chamber. The filter chamber proper is lined by the mucoprotein secretion of the cuboidal cells. This secretion appears to bind potassium ions. ATPase and alkaline phosphatase cannot be detected in the filter chamber epithelia. The structure and cytochemistry of the filter chamber suggests that water flows from filter chamber proper to midgut and Malpighian tubules by passive osmosis. This may be facilitated by ion binding in the filter chamber proper and by hydrostatic pressure engendered by contraction of the muscular coat. The Malpighian tubules appear to be structurally and chemically adapted for ion secretion by active transport and possibly for reabsorption in the Malpighian duct segment.     

Gut ultrastructure of the appendicularian Oikopleura dioica (Tunicata)

Abstract. The appendicularians, planktonic tunicates, possess a specialized, external filtering system that captures food particles <1 μm in size. In this work the alimentary canal of Oikopleura dioica has been studied by serial sections of whole animals and ultrastructure. The gut includes a dorsal esophagus, a bilobed saccular stomach, and a curved intestine, divided into vertical, mid-, and distal intestine (or rectum). No multicellular glands or cellular proliferative centers were found. Three main cell types were recognized, ciliated microvillar cells, globular cells and gastric band cells, with specializations reflecting different physiological roles in the various regions. Ciliated microvillar cells, the most diffuse, are considered to be involved in food propulsion, fecal pellet formation, absorption, and nutrient storage. Pinocytotic features and vacuoles suggest that absorption of macromolecules and intracellular digestion occur in the globular cells of the stomach and rectum. The large gastric band cells of the left lobe have typical features of intense protein synthesis and probably produce enzymes for extracellular digestion. Diffuse interdigitations of many cells enormously increase the plasmalemma surface and may be involved in liquid/ion exchange. Despite the apparent structural simplicity of the gut epithelium, O. dioica efficiently processes food to fulfill the energy requirements of its exceptionally rapid life-cycle.     

short gastrulation, a mutation causing delays in stage-specific cell shape changes during gastrulation in Drosophila melanogaster

Mutations at the short gastrulation locus affect the timing of certain early morphogenetic events occurring during gastrulation in Drosophila melanogaster. Specifically, the invagination and subsequent closing of the posterior midgut and the anterior midgut appear to be delayed in these embryos. In addition, their germbands do not extent the full distance anteriorly on the dorsal side of the embryo. The dorsal cells are abnormally thick and fall into extremely deep dorsal folds as the germband extends. sog embryos continue development, but form disorganized first instar larvae. Normal sog expression is required in the zygote, but not in the mother for normal embryonic development and viability. Analysis of adult and larval gynandromorphs indicates that sog expression is required only in the ventral and/or anterior and posterior ends of the embryo, arguing that the dorsal abnormalities caused by the mutation are secondary consequences of defects elsewhere in mutant embryos.     

External morphology of the larvae of Ephydra (Hydropyrus) hians (Diptera: Ephydridae)

Scanning electron microscopy is used to describe the ultrastructure of morphological features related to feeding, locomotion, respiration, and pigmentation in second- and third-instar larvae of Ephydra hians (Ephydridae: Diptera). Using larvae fixed with the head extended, the ultrastructural details of the mandibles, maxillary papilli, and antennae are described for the first time. The larvae possess finger-like projections at the anterior spiracle and a distensible bifurcated siphon at the posterior spiracle. Slit-like openings are found on each. The second- and third-instar larvae can be differentiated on the basis of dorsal pigmentation associated with extensive patches of spines. The larvae have regional specializations in spine distribution, particularly on the ventral and dorsal body surfaces, and the prolegs. © 1994 Wiley-Liss, Inc.     

On the Fine Structure of Rastrognathia macrostoma gen. et sp.n. Placed in Rastrognathiidae fam.n. (Gnathostomulida)

Rastrognathia macrostoma gen. et sp.n. is described as belonging to Rastrognathiidae fam.n. The ultrastructure is described, especially the nervous and digestive systems and the epidermal specializations. In the epidermis the single cilia belong to the choanocyte-like cell type. An epidermal sensory cell with several cilia is described. The intraepithelial nervous system shows an orthogon pattern. Two ventral nerve trunks unite into a buccal ganglion. The jaws are secreted by the buccal epidermis. The endoderm consists of a single cell type. The food consists of bacteria. The mesodermal elements consist of six main longitudinal muscles.     

Morphology and ultrastructure of the alimentary canal of the cicada Platypleura kaempferi (Hemiptera: Cicadidae)

The alimentary canal of cicada Platypleura kaempferi is described. It comprises the oesophagus, filter chamber, external midgut section and hindgut. The elongate oesophagus expands posteriorly, with its posterior end constricting to become a bulb. The filter chamber consists of two parts: a very thin sheath and a filter organ. The filter organ is composed of the anterior and posterior ends of the midgut (internal midgut section), and the internal proximal ends of the Malpighian tubules. The external midgut section differentiates into a collapsed sac and a midgut loop. The latter is divided into three distinct segments. The hindgut contains a dilated rectum and a long narrow ileum. The distal portions of the four Malpighian tubules are enclosed in a peritoneal sheath together with the distal ileum before reaching to the rectum. Ultrastructurally, the oesophagus and the hindgut are lined with a cuticle. The filter chamber sheath consists of cells with large irregular nuclei. Filamentous substances coat the microvilli of the cells of the internal midgut section. The posterior end of the midgut comprises two types of cells, with the first type of cells containing many vesicles and scattered elements of rough endoplasmic reticulum. The anterior and posterior segments of the midgut loop cells have ferritin‐like granules. The ileum cells have well‐developed apical leaflets associated with mitochondria. Accumulations of virus‐like particles enclosed in the membrane are observed in the esophagus, conical segment, mid‐ and posterior segments of the midgut loop.     

Microscopic anatomy and ultrastructure of the digestive system of three Antarctic shrimps (Crustacea: Decapoda: Caridea)

Mandibles and stomachs of three Antarctic shrimp species (Nematocarcinus lanceopes, Notocrangon antarcticus, and Chorismus antarcticus) were investigated by means of scanning electron microscopy. Transmission electron microscopy of the midgut glands was applied to find out the nutritional status of the animals, which contained a broad variety of food items in their stomachs. In contrast to the Antarctic krill, the three carideans possess a dual filter system; primary filters in the cardia and secondary filters in the pyloric chamber. Morphologically, the mandibles and stomachs of the three species vary considerably; however, their food items are similar. We conclude that stomach contents do not really reflect the different modes of life or habitat preferences of the investigated species.     

Morphological specializations of the digestive tract of Zacryptocerus rohweri (Hymenoptera: Formicidae)

Light, scanning, and transmission electron microscopy are used to examine the morphology and ultrastructure of the peculiar digestive tract of the turtle ant, Zacryptocerus rohweri. The proventriculus is heavily sclerotized and covered with clusters of small spines. Narrow spine-lined channels converging at the opening to the midgut act as a fine filter of food; particles >12.5 μm are unable to pass through the proventriculus. In the midgut, ultrastructural study reveals bacteria among the microvilli of midgut epithelial cells. The hindgut of Z. rohweri consists of an enlarged, dark-colored pouch filled with masses of bacteria of three major morphotypes. A thick layer of circular muscle and deep infoldings of the epithelium greatly increase surface area for absorption. Newly emerged individuals appear to acquire these microorganisms by soliciting material from the abdomen tip of other older workers in the colony. Whether or not the hindgut bacteria are true symbionts is unknown; their acquisition and presence suggest that they may supplement the ants' limited, liquid diet by supplying essential amino acids and other nutrients. J. Morphol. 234:253–262, 1997. © 1997 Wiley-Liss, Inc.     

The sorting of food particles by Abra sp. (bivalvia: tellinacea)

The selection of food particles by Abra tenuis (Montagu), A. alba (Wood) and A. nitida (Müller) has been investigated.Material for ingestion smaller than 30μm is not selected according to size by A. tenius. Particles smaller than 0.5 μm are retained by the palliai organs and ingested but only particles larger than 1 μm appear to be retained with an efficiency approaching 100 %. The mesh size of the gill filter is found to be ≈ 3.0 × 0.5 μm.A. tenuis does not appear able to discriminate between particles smaller than 20 μm by their food value; however, relatively large silica particles which are devoid of food are partially rejected by the labial palps in favour of particles of similar size but having a bacterial coating. To a lesser extent the physical nature of particles seems to influence their selection by A. tenuis; clean angular particles are rejected in favour of clean rounded ones.Small, light particles appear to be transported on the gills directly to the mouth without coming into contact with the palps. Larger, heavier particles tend to drop from the gill to be caught by the palps which extend posteriorly to cup the entire ventral margin of the inner demibranch when the animal is feeding.The material ingested by A. alba is significantly finer than that taken into the mantle cavity indicating that the pallial organs actively select food by size. Selection of material for ingestion by size in A. nitida appears to be optional since only some of the animals examined had stomach contents significantly finer than material from the mantle cavity.