THE SKIN. 425 



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. 



Neinertean Affinities. Hubrecht and others have emphasised the 

 affinities between Nemerteans and Vertebrates. 



In Nemerteans : 



(1) The lateral nerve cords sometimes approach one another 



ventrally, and in rare cases dorsally. An approximation 

 dorsalwards, and union on that surface, would result in a 

 double dorsal nerve cord. 



(2) The firm dorsal sheath of the proboscis may correspond to a 



notochord. 



(3) The proboscis itself may correspond to the hypophysis or 



pituitary process characteristic of Vertebrate brains. 



(4) Two ciliated slits on the head may correspond to a pair of gill 



clefts. 



(5) There is no segmentation, but the branches given oft* from 



the nerve cords are sometimes serially arranged. 



It must be noted, that those who support these theories do not assert 

 that any Nemertean or Annelid is in the direct line of Vertebrate ascent. 

 They simply emphasise the demonstrable affinities. When these are 

 thoroughly worked out, it will be possible to say what Invertebrate types 

 are most nearly related to Vertebrates. 



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. 



The 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 significance, 

 and is frequently concerned in the excretion of waste and 

 the regulation of the body temperature. As one or other of 

 its many functions predominates, there are corresponding 

 structural modifications. One function which we find 

 oftenest emphasised at the expense of the others is that of 

 protection, and yet the fossil Glyptodon, the sluggish 

 Chelonia, the decadent Ganoids, seem to indicate that this, 

 in itself or in its correlated variations, is not conducive to 

 the continuance of the species. Indeed, the great develop- 



