142 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



and is also found in the English Channel and Chesa- 

 peake Bay. It is placed among the Chordata because 

 it is regarded by some zoologists as having a notochord, 

 gill slits, and a dorsal nerve cord. All of these are so 

 rudimentary that the position of the animal in the 

 scheme of classification is not yet definitely determined. 

 Other structural and developmental features ally Bala- 

 noglossus to the annelids, and the echinoderms, for 

 which reason the animal may be looked upon as an inter- 

 mediate form between these groups and the Chordata. 



SUBBRANCH AND CLASS 2. Urochorda 



The tunicates form a small and singular group of 

 animals now regarded as being the degenerate descen- 

 dants of primitive Chordata. 



FIG. 114. An ascidian. 



FIG. 115. Diagram of a tunicate, i, 

 inhalent opening; bs, branchial sac; 

 t, " tunic"; p, peribranchial cavity; 

 ce, esophagus; s. stomach; a, anus; 

 c, cloaca; h, heart; r, reproductive 

 organs; , nerve ganglion. 



They occur both as fixed 

 and as free swimming forms, 

 and as single individuals as 

 well as chains or groups of individuals. The most 

 common forms (the solitary Ascidians) are inclosed in a 

 leathery, elastic bag, one end of which is fastened to the 

 rocks, while the other has two orifices, for the inlet and 

 exit of a current of water for nutrition and respiration. 



