XIII 



PHYLUM CHORDATA 



13 



Affinities. — The inolnsion of iho Hcinielioi-da in the jtliylnm 

 Chonhita is an aiTanoomcnt the ])r(>]H'iL'iy of which is not uni- 

 versally adniiLk'd, and is carried out. hero 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 existino- ropresontativos of ancestral Chor- 

 dates, they are at least a greatly modifiedbraiich, taking its origin 



^i\--an 



-ret 



...Xst 



tnl 



Fi(i. 714. — Rhabdopleura. A, Entire zooid. n, mouth ; h, anus ; r, stalk of zooid ; <?, pro- 

 boscis ; c, intestine ; f, anterior region of trunk ; f/, one of the tentacles. (After llay Ijankester.) 

 H, Diagram of the organisation : median longitudinal section, seen from the left. a. arm ; 

 an. anal prominence ; col. collar ; col. ne. collar-nerve ; c. g. cardiac sac ; int. Intestine ; 

 in. mouth; ntc. " notochord " ; «■. oesophagus; pr. proboscis ; ^)r. c. proboscis-ccelome ; ret. 

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

 Schepotieif.) 



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 



