THE ANCESTRY OF THE VERTEBRATES 



663 



The relations of the thyroid gland in cyclostomes, and its derivation 

 from a hypostyle-like structure in the embryo, strongly suggest that 

 vertebrates came from an animal like the cephalochordates and uro- 

 chordates, in which food was drawn by ciliary action into an enlarged and 

 elongated pharynx and collected by the mucus-secreting endostyle. 



OPENING OF PERIBRANCH.CAV. 

 ! /ANUS 



A. LARVAL UROCHORDATE 



/DORSAL NERVE 



GILL SLITS' 



ANUS 

 POSrn OPENING OF 

 PERIBRANCHIAL CAVITY 



B. CEPHALOCHORDATE 



,DORS. KtRVE CORD 



_^„--,-/^ / \ \. ,WF~ ^INTESTINE ANUSi 



THYROID' l-fciLL SUTS ^HEART OLIVER 



C. VERTEBRATE CCYCLOSTOME) 

 Fig. 538. — Diagrams of A, Larval urochordate, B, Cephalochordate (Amphioxus) 

 and C, Vertebrate (Petromyzon), illustrating the fundamental characteristics of 

 chordates redrawn after Hesse-Doflein. If the so-called liver of the cephalochordate 

 be homologized with the intestine of urochordates the similarity of the two groups 

 appears to be greater. The endodermal strand in urochordates which is comparable 

 with the intestine of cephalochordates degenerates and disappears during metamorphosis 

 when the tail is lost. The branchial basket of both urochordates and cephalochordates 

 becomes surrounded by a peribranchial cavity. The evidence appears to justify the 

 assumption of a common genetic origin of the two groups. 



Appendicularia meets all the requirements of such a hypothetical ancestor 

 of chordates. 



Brooks suggests that gill slits originated in tunicates, not as respiratory 

 organs, but to allow water to escape from the pharynx, so that food which 

 passes to the stomach and intestine can be more concentrated. Dohrn's 

 hypothesis that gill slits are modified segmental nephridia lacks proof. 

 Dohrn himself admits that he is unable to "assign any reason (sic) why 

 segmental organs should unite with the gut." The suggestion obviously 

 is based on the assumption that vertebrates are derived from annelid 



