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 Amphzoxus is a Vertebrate very 
simple in its generad 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, Palzostraca, 
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. 
(t) 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 
Cheetopod 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- 
