The Early Vertebrates and the Fishes 



shape, and in its relations to other parts, to that of Amphi- 

 oxus. There is, however, an additional development of cartilage 

 in the region of the head, forming, in particular, a sheath- 

 like covering for the brain and also a kind of basket-work 

 support for the pharynx and gills. The original tube-like 

 form of the dorsal nerve cord is easily recognisable, but it 

 is distinctly distended at its front end into a brain, which 

 shows a division into a series of three distinct portions, called 

 respectively the fore, mid, and hind brain. This division, it 

 is interesting to observe, is the first process in the development 

 of the brain in all the higher animals. There is a pair of well- 

 developed hearing organs, and in the lamprey a pair of similarly 

 well-developed eyes. In the hag fish the latter are greatly 

 reduced, a condition which is explained by the creature's mode 

 of life. The nostril is unpaired, a condition which is probably 

 primitive. Respiration is carried out by means of gills, which are 

 situated in a series of six to eight pouches, each of which opens into 

 the gullet and again directly to the outside, the external openings 

 being an obvious feature of the animal. There is a very distinct, 

 simple heart, which pumps the blood to the gills, from whence 

 it is collected and distributed throughout the body. The 

 digestive canal is a simple tube, provided, however, with a liver 

 and a pancreas, the two most important digestive glands in the 

 higher animals. The sexes are separate, but traces of a previous 

 hermaphrodite condition seem to persist. Henceforward in the 

 vertebrate group the sexes are always separate. A character of 

 the sex organs which is to be regarded as primitive is that they 

 are unconnected with the excretory system, whereas in the higher 

 vertebrates the two systems are always strangely interconnected. 

 As in all the higher types, there is but one pair of male or female 

 reproductive glands. Finally, the round-mouths differ markedly 

 from the lancelet in the structure of the skin. In the latter animal 

 the skin is composed of a single layer of cells. In the former it 

 consists of an epidermis, some three or four layers thick, and an 

 underlying cuticle or * true skin ' ; in other words, the skin has 

 the same general structure as that of the higher types. The 



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