456 STRUCTURE OF VERTEBRATA. 



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 must admit that these are specialised not 

 very primitive types. 



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 very obscure. 



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

 brates remains unknown. At the same time, it is useful to inquire 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, etc. 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. 



(1) 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 possible that the tubular 

 character of the spinal cord and brain is the necessary 

 result of its mode of development, and without much 

 morphological importance. 



(4) Segmentally arranged ganglia about the appendages of some 



Chretopod worms may correspond to the branchial and 

 lateral sense organs of Ichthyopsida, and the ganglia asso- 

 ciated with some of the nerves from the brain. 



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



p. 468) is suggestive of the way in which 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 Chtetopods, little importance can 

 be attached. 



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

 culty, even if we grant that no emphasis is to be laid on the tubular 

 nerve-cord of Vertebrates, and admit the possibility of an inversion 

 bringing the ventral nerve-cord to the dorsal surface. 



Nemertean affinities. Hubrecht and others have noted certain re- 

 semblances between Nemerteans and Vertebrates. 

 In Nemerteans 



(i) The lateral nerve-cords sometimes approach one another 

 ventrally, and in rare cases dorsally. An approximation 



