XV] 



INTERNAL ANATOMY 



373 



-9 



--8 



--7 



A 



tentacles can be protruded between the valves of the shell, and thus 

 the area they affect is enlarged. The lip may be compared to the 

 epistome of the Phylactolaematous Polyzoa (p. 377). 



The mouth leads into a simple stomach which ends in a short 

 intestine. Both stomach and intestine are ciliated. A digestive 

 gland called the liver consisting 

 of a number of branching tubes 

 opens on each side into the 

 stomach, and as is the case in 

 the Crustacea much of the di- 

 gestion takes place inside these 

 glands. In the genera of Brach- 

 iopoda which have a hinged 

 shell the intestine ends blindly, 

 but in those which have no 

 hinge there is an anus which 

 may open in the middle line as 

 in Crania, or on the right side 

 of the body as in Lingula. 



On the dorsal surface of the 

 stomach is a small, muscular, 

 contractile vesicle, the heart 

 (Fig. 175). This gives off a 

 number of vessels, amongst 

 others one which passes to 

 each tentacle, an indication 

 that the tentacles have a 

 respiratory function. 



The chief part of the nervou s 

 system retains its primitive re- 

 lation to the ectoderm. Just 

 in front and just behind the 

 mouth there are thickenings of 

 the ectoderm forming a supra- 

 and sub-oesophageal ganglion 

 respectively, the latter con- 

 trary to the usual rule being 

 much the larger (Fig. 176). 



m . , i i , which has separated from shell during 



They are connected by two the process of decalcification. 



lateral cords and give off a 



number of nerves, one of which runs to each tentacle. No sense- 



Fio. 176. A longitudinal vertical median 

 section througii Argiope neapolitana. 



Ventral shell, 

 blood-vessel, 

 nerve-ganglion. 

 5. Stomach. 6. 

 of blood-vessels. 

 on dorsal shell. 



. Canal containing 



i. Sub-oesophageal 



4. Mouth. 



Stalk. 7. Plexus 



8. Median crest 



9. Membrane 



