476 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 farther back. For, while many 

 of their alleged Vertebrate characteristics are debatable, one cannot 

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

 affinities of the Enteropneusta with Invertebrate types are quite 

 obscure. 



We have, in fact, to acknowledge that the pedigree of Vertebrates 

 remains unknown, though alleged affinities have been discovered 

 among Annelids, Nemerteans, Arachnids, Crustaceans, Palseostraca, 

 etc. There is almost no great class of Invertebrate Metazoa whose 

 characters have not been ingeniously interpreted so as to reveal affinities 

 with Vertebrates. It will be enough to select one illustration. 



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 ancl brain is the necessary 

 result of its mode of development, and without much 

 morphological importance. 



(4) Segmentally arranged ganglia about the appendages of some 



Chaetopod 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 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 and well-developed body 

 cavity of many Annelids little importance can be attached. 



STRUCTURE AND DEVELOPMENT OF VERTEBRATES 



Having separately discussed the Hemichorda, Urochorda, 

 and Cephalochorda, we propose in this chapter to discuss 

 the general structure of Craniata and the development of 

 some of the important organs. 



Skin. This forms a continuous covering over the surface 

 of the body, serves as a protection to the underlying tissues, 

 in some instances retains its primitive respiratory sig- 



