226 PHYLUM ANNELIDA. 



Food canal. (i) The buccal cavity is protrusible as a 

 " proboscis" or introvert, which grips the sand, and bears 

 internal papillae with chitinous tips. The protrusion is due 

 to the pressure of the ccelomic fluid, while special muscles 

 bring about retraction. (2) The gullet has smooth walls, 

 and bears a posterior pair of glands, which secrete a 

 yellowish fluid, probably digestive. (3) The gastric region, 

 from the heart to the twelfth or thirteenth notopodium, is 

 covered with yellow cells and many blood vessels, and has a 



FlG. Il8. Cross-section ol Arenicola. After Cosmovici. 



E., Epidermis; c.m., circular muscles; /./;?., longitudinal muscles; 

 b.c., body cavity; gl. y gill; jr., setae; ./., nephridial pore; 

 a,br.) afferent branchial ; e.br., efferent branchial ; n. t ventral 

 nerve-cord, with blood vessels above ; d.v., dorsal vessel; /.z>., 

 lateral vessel ; s.z'.v., sub-intestinal vessels; z/.z>., ventral vessel; 

 > gut. 



median-ventral ciliated groove. (4) The intestinal region is 

 much folded, " in a concertina-like manner," by the caudal 

 septa, and is full of sand, from which the nutritive matter- 

 has been absorbed. The anus is at the very end. 



Body cavity. This is spacious, except in the tail 

 region, and contains a viscous coelomic fluid. Anteriorly 

 there are three transverse, partly muscular, septa or 

 diaphragms which moor the gullet. The first of these 

 diaphragms bears a pair of small pouches. Behind the 



