264 



THE BIOLOGY OF MARINE ANIMALS 



gastropods (Crepidula, Pterocera). This is a hyaline rod extending into the 

 stomach where it undergoes dissolution, thereby releasing its enzymes (22, 

 35, 48). 



The midgut gland of cephalopods (Loligo, AUoteuthis) produces digestive 

 enzymes. The gland consists of two recognizably distinct parts known as 

 liver and pancreas, both of which discharge into the caecum by a hepato- 

 pancreactic duct. In structure they present the appearance of ramifying 

 tubules ; histologically, the secreting cells are distinctly different in the two 



tdc 



em 



\A * *»»„■** 



teb 



*W^a. jS r tH*&&, 



ss 



■' *i'i 



'v.,;" 





ac 



f 



a/T7 



vea 



ea 



Fig. 6.6. Section through a Tubule of the Digestive Gland 

 of Philine quadripartita, an Omnivorous Tectibranch 



Enzymes are secreted by the digestive gland, and digestion is completed intracellu- 

 lar^, am, amoebocyte; ct, connective tissue; dc, digestive cell; ea, eb, excretory cells; 

 em, excretory masses from excretory cells;/, fat droplet; ss, secretory spherules; tdc, teb, 

 tips of digestive cells cut off from the epithelium; vea, vacuole with excretory material. 

 (From Fretter (24).) 



regions. Proteolytic, amylolytic and lipolytic enzymes are produced by the 

 gland. Preliminary digestion of foodstuffs takes place in the stomach, into 

 which hepatopancreatic secretions are passed from the caecum, and 

 digestion is completed in the latter. The digested food is absorbed in the 

 caecum and intestine. In Octopus enzyme secretion and absorption both 

 take place in the liver (16, 16a). 



Digestive diverticula are present in some of the lower chordates. Secre- 

 tion is confined to the stomach in some tunicates, e.g. Ciona, but in Boltenia 

 and Tethyum the gut diverticula are the primary organs of secretion. 



