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Changes associated with feeding in the histological and cytological structure of the digestive gland of the loliginid squid Sepioteuthis lessoniana were examined, along with the nature of both the intracellular and extracellular enzymes produced by the gland. The timing of the release of the extracellular enzymes during the digestive cycle was also determined using a quantitative experimental program. Like that of all coleoid cephalopods, the digestive gland was characterised by one type of cell with several functional stages. As is the case for other loliginid squids, however, the digestive cells did not contain the large enzyme-carrying boules that characterise the digestive glands of most cephalopods. Instead, smaller secretory granules were found in the digestive cells and these may be the enzyme carriers. The prominent rough endoplasmic reticulum, large mitochondria and active Golgi complexes present in the digestive cells are characteristic of cephalopods and indicate a high metabolic activity. Like that of other cephalopods, endocytotic absorption of nutrients and intracellular digestion occurs in the digestive gland of this squid. From quantitative and qualitative examinations of structural changes in the digestive gland of S. lessoniana after feeding, a schedule of its function during the course of digestion was proposed. This indicated that digestion was very rapid, being completed in as little as 4 h in S. lessoniana. Extracellular digestive enzymes were only released after the first hour following feeding, which implies that they are stored in the stomach between meals to increase digestive efficiency.

The structural and functional aspects of the digestive gland in the endosymbiont-bearing bivalve Loripes lucinalis were examined. Specimens were collected over a one year period (September 1991 to September 1992) from the Moulin Blanc beach, Bay of Brest, (Brittany, France). The two apertures leading from the stomach into the digestive diverticula are very large as are the digestive ducts and tubules. The main and secondary ducts possess no ciliated tracts and present a definite brush border. The structural aspects of the cells lining the ducts, in addition to the presence of abundant acid phosphatase activity, suggest that these cells are actively involved in absorption and intracellulgr digestion of food material. Lipid and glycogen are abundant throughout the digestive diverticula suggesting that this organ is the site of significant energy stores. The tubules contain the two classical cell types, namely the secretory and digestive cells. Also present within the tubules are extremely large acid polysaccharide-filled vacuoles which originate from the lysis of digestive and/or secretory cells. These vacuoles suggest an extensive process of tubule autophagy and regeneration. The intestine is lined with cilia and microvilli. The cytological features of the intestinal cells, together with an abundant acid phosphatase activity, suggest that these cells are capable of absorption and intracellular digestion. The wet weight of the gonad-digestive gland complex varied throughout the year between 12 and 61.2 mg with an average wet weight of 31.8 ± 8 mg. Following spawning, total wet weights dropped by 50%. The α-amylase activity was measured and an average β-amylase to protein ratio of 5.34 ± 1.09 U/g protein was obtained. It was observed that amylase activity expressed per g protein may vary with the animal's physiological status. The temporal evolution of absolute α-amylase activity throughout the sampling period revealed that the variations in activity were dependent on a combination of environmental factors, including temperature and food levels. Digestive activity, however, did not seem to correspond to reproductive efforts.

Amylase, laminarinase, and cellulase activities in the crystalline style were assayed in vitro 18°C at pH 4–8 for the suspension-feeders: Crassostrea virginica (Gmelin), Rangia cuneata (Sowerby), andv Geukensia demissa (Dillwyn), and the deposit feeders, Macoma balthica (L.), and M. mitchelli Dall. Interspecific differences were considerable, but detectable amylase, carboxymethyl-cellulase, and laminarinase activities were found for all five species. At pH 6, α-amylase activity accounted for between 30 and 70% of the total activity of the three carbohydrases, cellulase for 15–27%, and laminarinase for 14–47%. The specific activities for the three carbohydrase classes at pH 6 were similar in the two deposit- and the three suspension-feeders. Species-specific catalytic activity for one enzyme class was correlated with activity levels for the other two enzyme classes. Enzyme optima were obtained at pH values which were more similar for one enzyme class obtained from different species than for the three enzyme classes obtained from the crystalline style of one particular species. For any particular species, maximum activities for crystalline style carbohydrase and the corresponding digestive gland carbohydrase were obtained at pH values which differed no more than 0.5 pH units, while the total pH range was 6.5–7.5 for α-amylases and 5–6 for laminarinases. In 13 cases out of 15, the specific carbohydrase activity of the crystalline style exceeded that of the corresponding digestive gland. We suggest that crystalline style carbohydrases play an important role not only by catalyzing extracellular digestion in the stomach, but also intracellular digestion in the digestive diverticula.

A study has been made of the processes of extra- and intracellular digestion in the Pacific oyster, Crassostrea gigas (Thunberg) over two 24-h cycles in winter and summer. The results show that the digestive processes are discontinuous and related to the tidal cycle. Variations in tidal height resulting from a diurnal inequality of the tide affect both the relative dissolution of the crystalline style and the relative lengths of the constituent phases of the intracellular digestive process in the digestive diverticula. On a seasonal basis the style is present for a greater length of time in winter and, conversely, remains dissolved longer in summer. A seasonal variation in the structure of the digestive tubules has also been found. The results confirm conclusions reached earlier that the processes of extracellular digestion in the stomach and intracellular digestion in the digestive diverticula of intertidal bivalves are both discontinuous, alternate, and irrevocably interrelated since breakdown of the digestive cells of the digestive diverticula following intracellular digestion results in the dissolution of the crystalline style. The arrival of food in the stomach has a minimal effect upon the style. Moreover, the cytological structure of the digestive diverticula of C. gigas undergoes a series of synchronized changes related to the tidal cycle.

The orientation of individuals in two populations of Pecten maximus L., from the west coast of Ireland, shows that they have a marked preference to face directly into a tidal flow. In both localities examined there was a reversal of tides and the members of the populations were divided equally for flood and ebb tides. Twenty-four hour in situ studies of the animals were made and at all stages of the tide individuals facing either due east or west were observed. A cyclical feeding pattern imposed by the reversal of tidal flow is proposed. The pH of various parts of the digestive tract was investigated and showed a wide range of values. The most acid region was that of the stomach. The variations in pH of all of the regions of the gut examined throughout a 24-h period closely followed the pattern of the tidal cycle. Histological analysis of the stomach and digestive diverticula of representative samples of scallops taken at regular intervals over the 24-h periods clearly indicated a diphasic pattern of digestion within the tubules of the digestive diverticula. A close correlation between the phases of intracellular digestion, the pH variations in different regions of the gut, and the tidal cycle indicate distinct feeding cycles in Pecten . Those scallops facing into the ebb current show the same diphasic patterns as those individuals facing the flood current, but are 6 h out of phase. The cycle is considered as a duplication of a diphasic feeding pattern. The tubules themselves undergo a digestive process similar to that in Lasaea rubra (Montagu). The digestive cells phagocytose food material, begin intracellular digestion, and increase in size until they obscure the lumina. The dispersal of waste material and residual bodies is accomplished dramatically by a dehiscence of the tubule cells together with a loss of both digestive and crypt cells. New tubules are regenerated from the apices of those tubules breaking down. In any section of the diverticula tubules in two different conditions are found. The cycle of digestion takes 24 h and in order to facilitate feeding at each 12-h tidal cycle the tubules are equally divided into two phases with one 12 h behind the other.

Histochemical investigations have been made on the nature and location of digestive and other enzymes in the entozoic hoplonemertean Gononemertes australiensis Gibson. On the basis of physiological and morphological evidence it is concluded that Gononemertes , despite its entozoic existence, possesses carnivorous feeding habits and probably acquires its food by temporarily emerging from its ascidian host. The digestive physiology is typically hoplonemertean in form; extracellular proteolysis is accomplished by cathepsin C-like endopeptidases secreted from gastrodermal columnar cell vesicles, following which phagocytosis occurs and digestion is completed intracellularly by proteases, esterases, and upases acting in harmony. Intracellular digestion is initially acidic and finally alkaline and involves acid and alkaline phosphatases at the appropriate stage. Carboxylesterase-like enzymes, functional at the intracellular level, have not previously been reported from nemertean worms. Food reserves consist principally of fat in the gastrodermis but small amounts of glycogen are also present. The occurrence of enzymic activity at several locations other than the gut is recorded. Among these, the arylamidases of the rhynchocoel fluid and epithelium are of particular interest in that they lend strong support to a hypothesis concerning the manner whereby the proboscis receives nourishment.