XIII 



PHYLUM CHORDATA 



13 



Affinities. The inclusion of the Hemichorda in the phylum 

 Chordata is an arrangement the propriety of which is not uni- 

 versally admitted, and is carried out here partly to obviate the 

 inconvenience of erecting the class into a separate phylum. On 

 the whole, however, there seems to be sufficient evidence for the 

 view that, if not the existing representatives of ancestral Chor- 

 dates, they are at least a greatly modified branch, taking its origin 



-an 



-rcl 



-si 



irib 



FIG. 714. Rhabdopleura. A, Entire zooid. , mouth ; 6, anus ; c, stalk of zooid ; d, pro- 

 boscis ; e, intestine ;/, anterior region of trunk ; g, one of the tentacles. (After Ray Lankester.) 

 B, Diagram cf the organisation : median longitudinal section, seen from the left. a. arm ; 

 an. anal prominence ; col. collar ; col. ne. collar-nerve ; c. s. cardiac sac ; int. intestine ; 

 m. mouth; ntc. " notochord " ; oe. oesophagus; pr. proboscis ; pr. c. proboscis-coelome ; ret. 

 rectum ; at. stomach ; te. tentacles ; tr. c. trunk-coelome ; v. n. ventral nerve. (After 

 Schepotieff.) 



from the base of the chordate tree. The presence of the pre- 

 sumed rudimentary representative of a notochord and of the gill- 

 slits seems to point in this direction. It should, however, be stated 

 that by some of those zoologists by whom the members of this 

 group have been most closely studied, their chordate affinities are 

 altogether denied. If the Hemichorda are primitive Chordates, 

 the fact is of special interest that they show remarkable 



