CHAPTER IX 



PISCES 



Phylum VBBTEBRATA 



Sub-Phylum CEANIATA Type 



Class PISCES Eaia 



AMPHIOXUS, as we have seen, has no distinct brain, nor is a 

 skull developed. The only skeletal tissue is the notochord 

 and the bars which distend the mouth and support the delicate 

 gills, and it is of a simple gelatinous character. The Craniata 

 have a brain which stands out from the rest of the central 

 nervous system, and a skull. The skull in the lowlier members 

 of the series is made of cartilage, and in the higher it begins 

 in cartilage and is more or less completely replaced by bone. 

 Except the lampreys and hags, the Craniates possess typically 

 two pairs of limbs, and, however modified, these can always 

 be recognised as pectoral and pelvic. In the limbs a carti- 

 laginous skeleton is developed which in the higher Craniates 

 is replaced by a skeleton of bone. Similarly, a jointed skeleton 

 of cartilage, the vertebral column, which may also give place to 

 bone, is developed around the notochord. 



The skate, genus Kaia, does not correspond to our ideas 

 of the form of a fish. Most fish have a fusiform shape. The 

 head passes into the greatest thickness of the body, and the 

 body tapers again to form the tail. Locomotion is produced 

 by the action of the long muscular tail with the aid of the 

 caudal fin, which is a vertical expansion of its terminal margin.' 

 In such a fish it will be seen that the body may be divided 

 into head, trunk or body, and tail. The trunk bears the 

 paired pectoral and pelvic fins, and in addition there are median 



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