202 Elementary Zoology. 



has a dorsal nervous system, swelling out in front to something 

 resembling a brain. This nerve-tube is hollow. There are 

 numerous vertically elongated gill-slits, but the young Amphi- 

 oxus has much fewer, and is more like the Vertebrata in this 

 respect. Excretory organs are present as a series of paired 

 tubes opening into the coelom on the one hand, and into the 

 atrium on the other — the atrium being a late ingrowth from 

 the exterior which burrows its way among the organs of the 

 body. 



Class \. VERTEBRATA. 



The word " Vertebrata " is etymologically incorrect when 

 applied to the present group, for the lowest members of it, 

 the lampreys, have no vertebrae. The term " Craniata '' often 

 used, is a much more appropriate term. The Vertebrata, or 

 Craniata, are to be distinguished from all the other Chordata 

 by the fact that they possess a definite skull, which is either 

 cartilaginous or bony, or partly cartilaginous and partly bony. 

 The brain is well marked off from the spinal cord. The liver 

 is always a more or less complicated gland, not a mere sacular 

 diverticulum as in Amphioxus. The Vertebrata are divisible 

 into the six following sub-classes : — 



Sub-class I. Cyclostomata. — The lampreys and hagfishes 

 are so different from the true fishes that they form a definite 

 group apart from them. They have a persistent notochord 

 (round which, however, vertebras seem to have existed in the 

 extinct Paleeozoic PaltBOspondylus) and a smooth scaleless 

 skin (unless the Devonian fishes, Pteraspis, etc., are ultimately 

 proved to be Cyclostomes) ; the skull resem.bles in many 

 particulars that of the embryos of higher forms ; there are no 

 true branchial arches, but a basket-work of cartilages, which 

 strengthen the gills. The Cyclostomes have no limbs, unless 

 certain rudiments in the neighbourhood of the cloaca turn out 

 to be degenerate limbs of the hind pair. The alimentary canal 

 is straight, and the pancreas is absent. There are no teeth, 

 only horny plates, and the mouth is a sucking, not a biting 

 mouth. 



