310 CLASSIFICATION OF CHORDATA 



hollow nerve cord, and a somewhat doubtful notochord, while the 

 other class, the Pterobranchia, represented by the colonial 

 Rhabdopleura, have only the doubtful notochord and, in some 

 genera, a single gill slit. They are interesting only as clues in 

 speculation on the origin of the phylum. 



SUBPHYLUM II— CEPHALOCHORDATA 



Branchiostoma adequately illustrates the structure of the group, 

 which contains only one other genus. The notochord reaching to 

 the extreme anterior end, the numerous gill slits, and the atrium, 

 may be taken as characteristic. 



SUBPHYLUM III— UROCHORDATA or TUNICATA 



The only chordate feature possessed by most adults is the gill 

 slits, but in the larva there is a short notochord in the tail. The 

 nervous system is reduced to a ganglion, and the direction of flow 

 of blood periodically reverses. The adult is almost completely 

 surrounded by a case made of tunicin, a substance very similar 

 to cellulose in composition. Many adults are sessile, such as Ciona 

 on British coasts, and some are colonial, but the larvae are pelagic 

 and tadpole-like. 



These three subphyla are spoken of collectively as proto- 

 chordates. There remains : 



SUBPHYLUM IV— VERTEBRATA 

 This is dealt with more fully in later chapters. 



