DIGESTION 



261 



pteropods. Proteases and sucrases are secreted into the gut in other 

 gastropods (vide infra) (46). 



Limulus and pycnogonids among arthropods retain a degree of in- 

 tracellular digestion. The food of Limulus consists of polychaetes and 

 lamellibranchs which are torn up by the gnathobases of the walking legs 

 before being swallowed. Enzymes, including proteases, are released into 

 the stomach cavity but the breakdown of proteins is completed intracel- 

 lularly, within the cells of the digestive gland, by an intracellular dipepti- 

 dase. The food of pycnogonids consists of fluids, soft tissues and fine par- 

 ticles. Secretory cells in the midgut release extracellular enzymes, while 

 absorptive cells in the same region ingest materials and complete digestion, 

 especially of proteins, intracellularly. 



In addition to the gut epithelium proper, phagocytic amoebocytes also 

 ingest and decompose food particles in certain animals. 



Intracellular Digestion by Amoebocytes. Intracellular digestion of 

 food particles by wandering amoebocytes takes place in lamellibranchs 



Posterior Rectum 

 adductor / 



Aorta. Ventricle Auricle 



l, ver Ctenidium 



Intestine Caecum 



Fig. 6.4. Internal Anatomy of the Shipworm Bankia gouldi. Anterior Half 

 of the Body, with Left Valve, Mantle and Gill removed (after Sigerfoos, 



1907.) 



and echinoderms. Amoebocytes are numerous about the stomach, digestive 

 diverticula and midgut of bivalve molluscs, and it has been observed that 

 these cells pass through the epithelium of the gut into the lumen, where 

 they ingest food particles, and then return to the tissues to digest this 

 material. Under abnormal conditions the amoebocytes may pass through 

 the gill membranes and absorb food material from the mantle cavity. 



Digestive amoebocytes occur in the gut of filter-feeding lamellibranchs 

 but are absent in protobranchs and carnivorous septibranchs. The 

 digestive capacity of amoebocytes has been investigated in Ostrea, 

 Ensis and other bivalves: they are known to contain proteolytic, lipolytic 

 and sucroclastic enzymes, and are capable of absorbing glucose (26). 

 Teredo and Bankia (Teredinidae) are wood-borers, and the particles of 

 wood swallowed by the animal are passed into a specialized region of the 

 digestive diverticula : here they are taken up by amoeboid cells and attacked 

 by an intracellular cellulase (Figs. 6.4, 6.5). The tridacnids are peculiar in 

 that their amoebocytes house symbiotic zooxanthellae which are ultimately 

 digested by the host (p. 614). Yonge (72) has pointed out the important 



