662 



COMPARATIVE ANATOMY 



claimed to be the missing link. The pendulum of opinion, however, later 

 swung away from this view, when Dohrn and Ray Lankester made it 

 seem probable that tunicates are degenerate vertebrates. But . iks 

 (1893), after a thorough investigation of the anatomy and develop 

 of pelagic tunicates, stated that he was unable to find any t 

 degeneration in Appendicularia, a pelagic larval type of Urv 



PROBOSCIS PORE' 



NERVE STRAND 



.CONAO 



NOTOCHORO' MOUTH' I ^"*''^''* 



IPHARYCEAL POOCH 



CEPHALODISCUS. 

 Fig. 536. — A diagram of Cephalodiscus viewed from the left side as if in median 

 optical section. The presence of a notochord in the pre-oral lobe is one of the reasons 

 for placing this animal among hemichordates. While not regarded as a form "ancestral " 

 to vertebrates, cephalodiscus interests morphologists as a primitive chordate. (Redrawn 

 after W. Patten.) 



As a result, Brooks totally rejected the dogma that vertebrates are 

 modified annelids and the assumption that tunicates are degenerate 

 vertebrates. He regards the free-swimming appendicularia as the type 

 from which chordates have evolved, and thus revives the tunicate theory 

 of vertebrate ancestry. The many points of resemblance of appendicularia 



STATIC ORGAN V 



>ENDODERM STRAND 



PHARYNX- 

 GILL SLITS'" 



OCELLUS) (HE^T 



Fig. 537. — Diagram of a larval urochordate. The similarity of the larval uro- 

 chordate to the embryo of a cephalochordate (amphioxus) suggests that a form like 

 this lies near the main line of vertebrate ancestry. (Redrawn after von Beneden and 

 Julin modified.) 



to amphioxus, and the similarity of their development, give a strong 

 foundation to this hypothesis. Appendicularia, like the larvae of other 

 tunicates, is obviously a chordate with fundamental chordate traits — 

 notochord, hollow dorsal nerve cord, expanded "brain" vesicle, gill- 

 slits, endostyle, and ventral heart. Indeed, the resemblances between 

 tunicates and amphioxus led Minot to suggest that the urochordates and 

 cephalochordates should be grouped together as Atriozoa. 



