250 The Study of Animal Life part in 



firm collar behind the mouth, a part with numerous gill-slits hehinil 

 the collar, and finally a soft coiled portion with the intestine and 

 reproductive organs. The size varies from about an inch to 6 

 inches, the colours are bright, the odour is peculiar ; the sexes are 

 separate. But Balanoglossus is most remarkable in having a dorsal 

 supporting rod (like a notochord) in the "proboscis" region, a 

 dorsal nerve-cord running along the back and especially developed 

 in the collar, and a series of gill-clefts on the anterior part of the 

 food-canal. It is therefore difficult to exclude Balanoglossiis from 





Fig. 51. — Cephalodiscus, a single individual, isolated from a colony. It i-s much 

 magnified. (From Chambers's Encyclop. ; after Chai/cjigxr Report, by 

 M'Intosh and Harmer.) 



the Vertebrate series, and it is likely that the same must be said 

 of another strange animal, Cephalodiscus^ discovered by the Chal- 

 lenger explorers. 



2. Tunicates. — Hanging to the pennon-hke seaweeds which 

 fringe the rocky shore and are rarely uncovered by the tide, large 

 sea-squirts sometimes live. They are shaped like double-mouthed 

 wine - bags, 2 or 3 inches in length, and water is always being 

 drawn in at one aperture and expelled at the other. Usually they 

 live in clusters, and their life is very passive. We call them sea- 

 squirts because water may spout forth when we squeeze their 



