424 STRUCTURE OF VERTEBRATA. 



Ancestry of Vertebrates. 



It is not at present possible to trace the path along which Vertebrates 

 have evolved, though our faith in the doctrine of evolution — as a modal 

 theory of origins — ^leads us to believe that Vertebrates arose from forms 

 which were not Vertebrates. 



But, even when we recognise that Amphioxus is a Vertebrate very 

 simple in its general features, and that the Tunicata, especially in their 

 youth, are Vertebrates, we have to remember that degeneration seems 

 to have been by no means uncommon in the history of animals, and 

 that the types above mentioned may be less primitive than they seem. 



The Enteropneusta carry us a little further back. For, while many 

 of their alleged Vertebrate characteristics are debateable, one cannot 

 gainsay, for instance, the possession of pharyngeal gill slits. But the 

 affinities of the Enteropneusta with Invertebrate types are entirely 

 obscure. 



We have, in fact, to acknowledge frankly that the pedigree of Verte- 

 brates remains unknown. At the same time, it is useful to enquire into 

 certain convergences towards Vertebrate structure which are exhibited 

 among various sets of Invertebrates. 



In regard to these, speculation has been abundant. Alleged affinities 

 have been discovered among Annelids, Nemerteans, Arachnids. Crus- 

 taceans, -Sic. Indeed, there is almost no great class of Invertebrate 

 Metazoa whose characters have not been ingeniously interpreted, or 

 wrested, so as to reveal affinities with Vertebrates. It will be enough 

 to select two illustrations. 



Annelid Affinities. — Dohrn, Semper, Beard, and others, maintain that 

 Annelids have affinities with Vertebrates. 



(i) Both Annelids and Vertebrates are segmented animals. 



(2) The segmental nephridia of Annelids correspond to the primi- 



tive kidney tubes of a Vertebrate embryo. 



(3) The ventral nerve cord of Annelids may be compared (in 



altered position) to the dorsal nerve cord of Vertebrates. 

 Both cords are bilateral, and it is likely enough that the 

 tuljular character of the spinal cord and brain is the neces- 

 sary result of its mode of development, and without much 

 morphological importance. 



(4) Segmentally arranged ganglia about the appendages of some 



Chcetopod worms may correspond to the branchial and lateral 

 sense organs of Ichthyopsida, and the ganglia as.sociated with 

 some of the nerves from the brain. 



(5) The formation of the oral part of the pituitary body {see 



page 436) is suggestive of the way in w hich the mouth of 

 Annelids is sometimes formed. Perhaps the pituitary body 

 represents an old lost mouth and its ancient innervation. 



To minor points, such as the red blood, well developed body cavity, 

 and slight internal skeleton of some Chnetopods, little importance can be 

 attached. 



The absence of anything like gill slits in Annelids remains as a diffi- 



