154 ANIMAL BIOLOGY 



the .term tunicate refers to the usually tough tunic or 

 covering which envelopes the body. When the develop- 

 ment of these animals came to be studied, the surprising 

 fact was revealed that the early embryonic stages strik- 

 ingly resemble the corresponding stages of vertebrate 

 animals, and it was found also that in the larval period of 

 the tunicate there is a dorsal nerve cord, gill-slits which 

 open from the pharynx to the outside like those of fishes, 



FIG. 125. Internal structure of a tunicate. A, point of attachment; 

 BC, branchial or gill chamber into which water enters through the orifice 

 shown in the upper part of the figure. After passing through the numer- 

 ous gill slits in the wall of this chamber the water is discharged through the 

 orifice shown at the right of the figure; H, heart; /, intestine. (After 

 Herdman.) 



a ventral heart, and a notochord, or rod-like structure 

 which corresponds to the primitive spinal column of the 

 vertebrates. The tunicate larva is a free-swimming 

 animal possessing the essential features of vertebrate 

 structure. Sooner or later the larva settles down and 

 becomes attached by its head; the tail is resorbed, and a 

 complex metamorphosis ensues in which most of the 

 vertebrate characters are either lost or much obscured. 

 The adult tunicate is a degenerate animal, and were it 



