DIGESTION 



259 



style attack cellulose and starch, but digestion of other foodstuffs is mainly 

 intracellular, following phagocytosis in the digestive gland (28, 30, 70). 



Similar sorting mechanisms occur in the stomach or anterior intestine of 

 many herbivorous and microphagous gastropods (Vermetus, Struthiolaria, 

 Ap/ysia, etc.). In cephalopods (Sepia, Lo/igo, etc.) the stomach is a digestive 

 chamber. Connected with the stomach by a vestibule is a caecum, which 

 receives the ducts of the digestive glands. The caecum contains an elabor- 

 ate mucous and ciliary collecting mechanism, the function of which is to 



Midgut 



Style sac 



Duct 

 digestive 



diverticula 



Groove from 

 caecum 

 to midgut 



Oaspric 

 shield 



Ciliated tract 

 to caecum 



Caecum 



Duct of 



digestive 



diverticula. 



Oesophagus 



Fig. 6.3. Stomach of the Oyster, Opened out to Show 

 Ciliary Currents ( x 7.6). (From Yonge (70).) 



clear the nutrient fluid of all solid particles. Soluble material is absorbed, 

 while particles and mucus are carried to the intestine (16, 29, 35, 47). 



In most fishes the initial phases of digestion are completed in the 

 stomach, where food is reduced to a semi-liquid chyme. A strong sphincter 

 muscle closes the lower end of the stomach until the food reaches the 

 proper fluid condition for further treatment by the intestine, when the 

 valve periodically relaxes and allows some of the chyme to pass through. 



DIGESTION 



Chemical breakdown or digestion of foodstuffs may be intracellular or 

 extracellular. Among holozoic protozoans and sponges, prey or particul- 

 ate matter is ingested, and digestion takes place inside the cell. In the meta- 

 zoa intracellular digestion is found in coelenterates, ctenophores and tur- 

 bellaria, and also occurs in certain higher groups of animals in correlation 

 with their manner of feeding. 



