268 



THE STUDY OF ANIMAL LIFE CHAP. 



body-cavity develops as five pockets from the primitive 

 gut. In any case, it is difficult not to include Balano- 

 glossus and its allies in the Vertebrate or Chordate series, 

 and the same may be said of another strange animal, 

 Cephalodiscus, first discovered by the Challenger explorers. 

 2. Tunicates. Hanging to the pennon-like 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 i n 

 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 pas- 

 sive. We call them 

 sea-squirts because 

 Xvater may spout 

 forth when we 

 squeeze their bodies, 

 while the title Tuni- 

 cate refers to a 

 characteristic cloak 

 or tunic which en- 

 velops the whole 

 animal (fig. 85). 



There is not much 

 to suggest back- 

 bonedness about these Tunicates, and till 1866 no one 

 dreamt that they could be included in the Vertebrate 

 series. But then the Russian naturalist Kowalevsky 

 discovered their life-history. The young forms are free- 

 swimming creatures like miniature tadpoles, with a 

 dorsal nerve-cord and brain, a supporting axis or noto- 

 chord in the tail region, gill-slits opening from the 

 pharynx, a little eye arising as an outgrowth of the 

 brain, and a ventral heart. 



Fir/. 84. CEPHALODISCUS, A SINGLE INDI- 

 VIDUAL, ISOLATED FROM A COLONY. Ix 

 IS MUCH MAGNIFIED. 



(From Chambers's Encijclop. : after Chal- 

 lenger Report, by M'Intosh and Harmer.) 



