192 BRANCH CHORDATA 



is the collar, opening by two spores into the first gill-slit; the re- 

 mainder constitutes the flattened but nearly cylindric trunk. 

 There is no segmentation of the body. By alternately contract- 

 ing and dilating the proboscis and the collar the Balanoglossus 

 can burrow in the mud. 



Gill-slits. — On the dorsal surface of the anterior portion (the 

 branchial region) of the trunk is a double row of gill-slits which 

 increase in number throughout life. 



Digestive System. — The mouth is situated ventrally at the base 

 of the proboscis just within the collar, and from it the alimentary 

 canal extends to the posterior extremity of the body. "Into the 

 dorsal half of the anterior portion of the alimentary canal open 

 the internal gill openings." The hepatic cceca bulge out in ex- 

 ternal prominences in the middle part of the canal. The 

 anal opening is at the posterior end of the body. 



The notochord, "a blind tube surrounded by a tough mem- 

 brane, extends from the pharynx into the proboscis." There are 

 dorsal and ventral nerve strands connected by nerves in the 

 collar. 



A dorsal blood-vessel lies above the notochord. 



The larvae of some species so much resembles certain echino- 

 derms that the Balanoglossus was formerly placed with that 

 branch. 



SUB-PHYLUM AND CLASS II. UROCHOR'DA OR 

 TUNICATA 



This degenerate group is represented by minute animals a few 

 centimeters long and by some measuring several feet in length. 

 They are found singly or in string-like colonies which have been 

 developed from a solitary individual by budding, the two forms 

 thus giving rise to alternation of generations. Multiplication 

 is both sexual and asexual. They are hermaphroditic, but cross- 

 fertilization occurs. They are marine and most of them are 

 pelagic. 



The most common forms, the "sea-squirts" or ascidians (Fig. 

 155), are surrounded by a tough elastic bag, one end of which is 

 attached to stones. At the other end is a large round oral 

 aperture, for the inlet of water carrying food and air, and near it, 



