Observations on the feeding mechanism,structure of the gut,and digestive physiology of the european lobster Homarus gammarus (L.) (Decapoda: Nephropidae)

Investigations have been made on the feeding mechanism, structure of the gut, and digestive physiology of the European lobster Homarus gammarus (L.).Ciné-photography has shown that the mandibles do not possess a masticatory function, merely serving to grip food morsels during the tearing process effected by the pulling action of the third maxillipeds. The remaining maxillipeds, together with the maxillae, then direct food fragments to the mouth for ingestion.Ingestion is facilitated by mucoid secretions discharged from the oesophageal tegumental glands; the glands do not appear to produce any enzymes which directly contribute to the digestive processes.The hepatopancreas is the principal organ concerned with digestion. It possesses a complex tubular organization in which sequential cellular differentiation culminates in the discharge of enzymes from the B-cells for extracellular digestion in the cardiac stomach. The enzymes are synthesized within vacuoles contained in the B-cell precursors (F-cells) and are secreted in three bursts of activity at 0–15 min, 1–2 h, and 3.5–5 h after a meal. The initial secretory phase is holocrine. Extracellular digestion involves esterases, arylamidases, and lipases; endopeptidases have not been positively identified by histochemical means despite the fact that Homarus is a carnivore. There is an intracellular digestive phase, not previously described in decapod crustaceans, at the 7–9 h post-ingestive stage in the hepatopancreatic R-cells which is effected by arylamidases and lipases.Various phosphatase enzymes have been identified in the hepatopancreatic cells. Acid and alkaline phosphatases are apparently concerned with several stages in the digestive cycle, including enzyme synthesis and secretion, and the absorption of digestive products. Adenosine triphosphatase activity is primarily associated with granules located in the distal R-cell cytoplasm; the possible significance of these granules in the elimination of metabolic wastes is discussed. Acid phosphatases and esterases are present in the midgut epithelium. The possibility of a passive uptake of material from the midgut lumen is considered.Faecal material in the hindgut is bound by mucoid secretions derived from the tegumental glands of this alimentary region; the mucus may also assist in defaecation.A complete digestive cycle in Homarus occupies ≈ 12 h.Food reserves in the gut consist principally of fat deposits in the R-cells, but minute amounts of glycogen can also be detected.No evidence of calcium, copper or ferric iron deposition in any part of the alimentary tract was found.     

Oleospheres of the cave-dwelling shrimp Troglocaris schmidtii: A unique mode of extracellular lipid storage

The cave-dwelling shrimp, Troglocaris schmidtii, has a unique mode of lipid storage. The lipid lies extracellularly in specialized compartments of the hepatopancreas, named oleospheres. The lipid is synthesized in the R-cells of the hepatopancreatic epithelium and accumulates in lipid droplets which fuse to form bigger globules. Mature lipid globules display moderately electron dense centers probably comprising triglycerides, and a broad electron dense boundary presumably consisting of lipoproteins. The globules are discharged into the lumen of the hepatopancreatic tubules by a kind of apocrine secretion. There, they coalesce to form larger masses. Finally, these lipid masses are transported into the oleospheres through a valve-like structure. The continual accumulation of lipid results in a drastic expansion of the oleospheres up to 500 μm in diameter. The absence of food in the digestive tract and the inactivity of the digestive enzyme producing F-cells indicate that digestion is suspended in the period of oleosphere formation. The curious mode of lipid storage in T. schmidtii may represent an adaptation to the extreme environmental conditions of a cave.     

The influence of starvation and different diets on the hindgut of isopoda (Mesidotea entomon,Oniscus asellus,Porcellio scaber)

Summary Under conditions of food deprivation, the hindgut epithelium of the experimental animals (Mesidotea entomon, Oniscus asellus, Porcellio scaber) undergoes ultrastructural changes. After the application of different diets it was demonstrated that this part of the alimentary canal contains nutrients, though it is lined by a cuticle. Experimental evidence for the formation of glycogen from glucose offered as the only diet comes from autoradiographic experiments. Amino acids, too, were detected in the hindgut cells soon after refeeding. Lipids, on the other hand, which are first absorbed by the large cells of the midgut glands, were not found in the hindgut epithelium. The existence of lipid inclusions in the hindgut epithelium some weeks after refeeding, however, supports the hypothesis that lipids reach the epithelial cells of the hindgutvia midgut glands and hemolymph.     

Digestive anatomy of Halella azteca (Crustacea,Amphipoda)

The digestive tract of the freshwater amphipod Hyalella azteca is a straight but differentiated tube consisting of foregut, midgut, and hindgut divisions. The foregut is subdivided into a tubular esophagus, a cardiac stomach, and a pyloric stomach. The cuticular lining of the cardiac stomach is elaborated into a set of food-crushing plates and ossicles, the gastric mill, while the pyloric cuticle forms a complex straining and pressing mechanism. Nine caeca arise from the midgut, seven anteriorly and two posteriorly. Four of the anterior caeca, the hepatopancreatic caeca, are believed to be the primary sites of digestion and absorption. The remaining caeca may be absorptive, secretory, or both. The much-folded hindgut wall is capable of great distention by extrinsic muscle action for water intake to aid in flushing fecal material out of the anus; such action also may stimulate antiperistalsis by intrinsic rectal muscles.     

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.     

Microscopic anatomy and ultrastructure of the digestive system of Natatolana obtusata (Vanhöffen, 1914) (Crustacea, Isopoda)

The scavenging cirolanid isopod Natatolana obtusata was caught using primarily meat‐baited traps at a station in the Weddell Sea at Kap Norvegica. The hindguts of most specimens were densely stuffed with meat from the traps. The anatomy and the ultrastructure of different sections of the digestive system are described. The large and extremely expandable mouth opens into a short oesophagus which extends posteriorly into a small stomach. Lateralia and primary filter can be observed directly when the mouth opening is widened to its extreme. The anterior hindgut is extremely variable in its lumen and very distensible, depending on the nutritional condition. The cuticle of the hindgut is colonized by fungi. The length of the midgut glands varies with the nutritional conditions. Morphology and anatomy of the digestive system of Natatolana obtusata are discussed with the background of the lifestyle of the species and compared with other taxa.     

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.     

Functional morphology of the midgut of Aedes aegypti L. (Insecta,Diptera) during blood digestion

Morphometric analysis of the epithelial lining of the stomach of A. aegypti suggests that digestion of the first blood meal in the stomach of this species can be viewed as a series of phases that can be correlated with physiological data from the literature. In phase Ia (0-10 h after blood meal [abm]) the whorls of the rough endoplasmic reticulum unfold, the Golgi zones increase, and the basal labyrinth is enlarged. This coincides with processes of synthesis and secretion (e.g., peritrophic membrane, esterases and lipases) and transport by the stomach epithelium. In phase Ib (10-20 habm) the cellular parameters measured further increase, indicating high synthetic and secretory activities (e.g., digestive enzymes). In phase Ic (20-30 habm) cell structures involved in synthesis and secretion still exhibit high values coinciding with maximal activity of proteases in the gut. Enhanced surface area of microvilli, prominent lipid inclusions, and appearance of glycogen deposits in the gut epithelium suggest increased absorption, storage, and transport functions of the stomach cells. In phase II (30-36 habm) structural alteration points to a gradual shift from synthesis and secretion to absorption, partial storage, and transport of nutrients. In phase III (36-72 habm) the cellular apparatus is reduced concomitant with the ending of the digestive cycle. Lipid inclusions and glycogen deposits disappear from the stomach epithelium.     

Anatomical and histochemical features of the digestive system of Octopus vulgaris Cuvier, 1797 with a special focus on secretory cells

This study reports a detailed anatomical and histological study of the digestive system of Octopus vulgaris. Emphasis was placed on characterising the glands and glandular cells and their distribution throughout the digestive tract. The use of classic histological and histochemical techniques revealed two morphological types of glandular cells: granular and mucous. Moreover, the histochemical analysis indicated specialisation of mucous glandular cells in the buccal mass, the submandibular gland and the caecum for secreting acid and neutral glycoconjugates. The cells of the anterior salivary glands are specialised for secreting neutral glycoproteins, and those of the posterior salivary glands are specialised for granular and mucous secretion. The oesophagus, crop and stomach lack glandular cells, but both granular and mucous glandular cells are found in the intestine. An unusual structure resembling the typhlosole of bivalves is described for the first time in the intestine of O. vulgaris. The highly ciliated epithelium and location of the structure in the anterior part of the intestine suggest a possible role in bypassing the caecum, stomach and intestine. We discuss how these cells and organs contribute to the process of digestion in the light of the present histological and histochemical data and of previously published information on the morphology and physiology of digestion in the octopus.     

Histological and ultrastructural characterization of the alimentary system of solifuges (Arachnida,Solifugae)

Solifuges are voracious and fast predators. Once having captured a prey item, mostly small arthropods or even small vertebrates, they start feeding on their prey by constant chewing movements with their huge chelicerae. At the same time, they squeeze out the soft tissue that passes the anterior lattice‐like part of the mouthparts. The digestion of the food takes place in the midgut, which is anatomically highly complex. It consists of the midgut tube from which numerous prosomal and opisthosomal diverticula and tubular lateral branches arise. The dimorphic epithelium of the midgut tube and the diverticula is constituted of digestive and secretory cells. The digestive cells are characterized by an apical tubulus system and contain nutritional vacuoles, lipids, spherites, and glycogen. Secretory cells contain a huge amount of rough endoplasmic reticulum and secretory vacuoles. The lateral branches are ultrastructurally similar to Malpighian tubules and are likely involved in excretion. In contrast to the midgut, the epithelium of the hindgut consists of only one type of cell overlain by a thin cuticle. Digested residuals are stored in the hindgut until defecation. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Male reproductive tract and accessory glands of a stomatopod,Squilla holoschista

In S. holoschista, the vas deferens is differentiated into 3 regions based on the functional morphology of the epithelial lining. The proximal part with a highly secretory epithelial layer may be responsible for the secretion which is rich in protein, and moderately so in carbohydrate and lipid substances. The mid-region serves for storage of the spermatozoa. The distal part has specialized epithelial cells of a secretory nature. In addition, there are 4 typhlosole-like ridges fringed with cilia which probably aid in the conduction of the sperm cord during copulation. Secretions of the accessory reproductive glands contain a large number of discrete granules rich in mucoprotein. The principal function of this secretion appears to be the digestion of the sperm cord in the oviduct.     

The effects of naphthalene on the ultrastructure of the hepatopancreas of the fiddler crab, Uca minax

The hepatopancreas of the red-jointed fiddler crab, Uca minax, is a bilateral evagination of the midgut, composed of numerous blind-ending tubules. Groups of these tubules empty into collecting ducts which join to form the main hepatopancreatic duct. Ultrastructural examination of tubules from the hepatopancreas of adult fiddler crabs revealed four major cell types, designated as E, R, B, and F cells. The E cells were found at the apex of the tubule and were assumed to serve as meristematic tissue. The R cells were most numerous and were scattered along the length of the tubule. Characterized by extensive smooth and rough endoplasmic reticulum and abundant lipid and glycogen reserves, the R cell was assumed to function in absorption and storage of the organic products of digestion. The B cells were recognized by the presence of a single, large apical vacuole that likely functioned in the secretion of digestive enzymes into the lumen of the hepatopancreas. The F cells, which contained extensive amounts of rough endoplasmic reticulum, were believed to be responsible for the synthesis of digestive enzymes. Electron microscopy of the hepatopancreas of crabs exposed to naphthalene for 5 days revealed that those cells with abundant membrane lipids (F cells) and abundant storage lipids (R cells) were most altered while those cells having little membrane or storage lipids (B and E cells) were only slightly altered. Furthermore, alterations in the F and R cells were not uniform along the length of the tubule, but increased in severity toward the proximal end.     

Feeding and gut physiology in Acanthopleura spinigera (Mollusca)

The morphology and histology of the alimentary canal of the rock chiton Acanthopleura spinigera are described and the ability of regions of the gut to digest specific substrates investigated. The oesophagus is produced into a pair of thin-walled lateral pouches, the salivary glands or "sugar glands" which empty into the stomach. Folds of the capacious stomach are almost obscured by the large digestive gland over which is coiled the intestine. Histologically the gut consists of an outer layer of connective tissue, an inner muscular layer and a ciliated epithelium which varies in thickness from one region to the next. Proteases are most active in the stomach, digestive gland and anterior intestine at pH 6·5 and in the posterior intestine at pH 7·5-8·5. The digestion of lipoidal substance was greatest in the stomach and digestive gland and least in anterior intestine. There was little increase in the amount of digestion product obtained after 20 hours incubation. All regions of the alimentary canal and salivary gland were capable of digesting carbohydrates except that many low molecular weight carbohydrates were digested by salivary gland extracts only. The amylases were most active at pH 6–6·5. It is concluded that digestive enzymes are distributed throughout the intestinal tract but the amount of enzyme present varies from region to region, and is greatest just after feeding.     

Fine structure of the midgut gland of Phalangium opilio (Chelicerata,Phalangida)

Summary The fine structure of the midgut gland and the changes in composition associated with the digestive activity were examined in Phalangium opilio. In the epithelium four different types of cells are present: ferment cells, resorption cells, and digestion cells which probably turn into excretion cells, as can be seen by many intermediate stages. Ferment cells are found only in the midgut gland and in no other epithelia; therefore they should be regarded as a cell type. The relationship between digestion and resorption cells is not yet clear. No regeneration zone or single regeneration cells could be identified.The ultrastructural changes in these different cells during digestion are described, and their functional aspects are discussed. A hypothetical digestive cycle is constructed from these data. The results are compared with those on other chelicerate midgut glands.     

Digestive enzyme compartmentalization and recycling and sites of absorption and secretion along the midgut of Dermestes maculatus (Coleoptera) larvae

Bostrichiformia is the less known major series of Coleoptera regarding digestive physiology. The midgut of Dermestes maculatus has a cylindrical ventriculus with anterior caeca. There is no cell differentiation along the ventriculus, except for the predominance of cells undergoing apocrine secretion in the anterior region. Apocrine secretion affects a larger extension and a greater number of cells in caeca than in ventriculus. Ventricular cells putatively secrete digestive enzymes, whereas caecal cells are supposed to secrete peritrophic gel (PG) glycoproteins. Feeding larvae with dyes showed that caeca are water-absorbing, whereas the posterior ventriculus is water-secreting. Midgut dissection revealed a PG and a peritrophic membrane (PM) covering the contents in anterior and posterior ventriculus, respectively. This was confirmed by in situ chitin detection with FITC-WGA conjugates. Ion-exchange chromatography of midgut homogenates, associated with enzymatic assays with natural and synthetic substrates and specific inhibitors, showed that trypsin and chymotrypsin are the major proteinases, cysteine proteinase is absent, and aspartic proteinase probably is negligible. Amylase and trypsin occur in contents and decrease along the ventriculus; the contrary is true for cell-membrane-bound aminopeptidase. Maltase is cell-membrane-bound and predominates in anterior and middle midgut. Digestive enzyme activities in hindgut are negligible. This, together with dye data, indicates that enzymes are recovered from inside PM by a posterior-anterior flux of fluid outside PM before being excreted. The combined results suggest that protein digestion starts in anterior midgut and ends in the surface of posterior midgut cells. All glycogen digestion takes place in anterior midgut.     

Functional Morphology of the Introvert and Digestive System of Myzostoma cirriferum (Myzostomida)

Myzostoma cirriferum feeds by diverting food particles carried by the ambulacral grooves of its comatulid host Antedon bifida. When searching for food, the myzostome uses its protrusible introvert to fulfil two major functions: sensory perception and the capture of food particles. The digestive system is composed of four parts, viz. a pharynx, that is contained within the introvert, a stomach, a series of paired caeca and an intestine that lie in the myzostome's trunk. The pharynx is supplied with a thick muscle which, thanks to peristaltic movements, carries food particles from the mouth to the stomach. Both stomach and caecal cells are able to absorb dissolved nutriments and to store lipids, whereas intestinal cells are only capable of absorption. Due to the beating of their cilia, stomach cells also carry food particles into the caecal lumen, where they are subjected to endocytosis and intracellular digestion by caecal cells. Undigested food fragments eventually gather in a very large, apical vacuole, and the cell apices containing vacuoles are eliminated into the caecal lumen by an apocrinal process. Detached cell apices reach the stomach, where they are embedded in a matrix, together forming a spindle-shaped faecal mass that is expelled through the postero-ventral anus. The observed digestive process—entailing the regular elimination of the apical part of the caecal digestive cells—appears to be unique among the Spiralia.     

Functional adaptations of the digestive system of the carnivorous mollusc Pleurobranchaea californica MacFarland, 1966

The opisthobranch mollusc Pleurobranchaea californica feeds on whole organisms and the functional morphology of the digestive system reflects this behavior. By a rhythmic behavior involving well-developed extrinsic buccal muscles and hemocoelic fluid, the buccal mass is protracted to the tip of the everted oral tube. Here a series of repeated protractions and retractions of the intrinsic buccal muscles associated with the flat radular ribbon and jaws draws the prey into the buccal cavity and conveys it to the dorsal esophagus, where by peristaltic action it is passed to the expansible crop for storage. Prey entering the buccal cavity is mixed with acid from a large single gland and secretion from the paired salivary glands. Prey is retained in the crop over long periods of time while it is slowly broken down and passed via the stomach into the digestive glands. Special modifications that allow flexibility of the digestive organs include elongated salivary gland ducts with propulsive bulbs, long flexible nerve cords connecting the ganglia, a long, large muscular duct for storage of the acid secretion, large jaws for muscle attachment and grasping the prey, and a broad radular ribbon with many teeth that acts as a conveyor belt to move food. Additional modifications for handling whole prey include a buccal membrane that aids in maintaining hemocoelic fluid pressure, the extensive acid gland for immobilization of prey, and the expansible crop for storage of food.     

Digestive strategy of the asian colobine genusTrachypithecus

The Asian colobines,Trachypithecus obscurus andT. cirstantus, eat plant-based diets containing 55–80% leaves. The structural polysaccharides in leaves and other plant parts require microbial fermentation before they can be used as an energy source by the monkeys. The major compartments of the gastro-intestinal tract ofTrachypithecus are a voluminous haustrated stomach, a long small itnestine and capacious haustrated hindgut, all of which contribute to the digestive strategy of these two species. Results of digesta marker passage studies indicate there is prolonged retention of digesta for fermentation in both the stomach and haustrated colon. The digestive strategy of these colobines is defined as gastro-colic fermentation, unlike that of other forestomach fermenters in which the hindgut fermentation is of secondary importance.     

The functional morphology of the alimentary tract of barnacles (Cirripedia: Thoracica)

The alimentary tract of barnacles is made up of cuticle-lined foregut and hindgut with an intervening U-shaped midgut associated anteriorly with a pair of pancreatic glands and perhaps midgut caeca. Epithelial salivary glands secrete acid mucopolysaccharide, glycoprotein or both. Cells of all the midgut regions are capable of absorption which is carried out mainly by the anterior midgut and caeca. Midgut cells of Balanus balanoides (L.) show a seasonal variation in the distribution of intracellular lipid droplets. Midgut cells rest on an elastic basal lamina and secrete a peritrophic membrane which contains mucopolysaccharide and protein. Cells of the stratum perintestinale connect with the midgut epithelial cells via cell processes which probably translocate absorbed materials. Glycoprotein globules and lipid droplets accumulate in the body parenchyma of B. balanoides and are transported to the ovaries to form yolk (glycolipovitellin). The pancreatic gland cells of all barnacles are active secretory cells secreting proteinaceous material (probably digestive enzymes).