CLASSIFICATION AND EVOLUTION 517 



complicated coelom, and with calcareous 

 plates in the dermis. 



Starfish, Sea-urchins, etc. 

 7. Phylum Chordata. 



Bilaterally symmetrical, usually segmented 

 Triploblastica, with a closed blood-vascular 

 system, a spacious ccelom, a hollow, dorsal 

 central nervous system, a notochord, and 

 gill-clefts. 

 a. Subphylum Cephalochorda. 



Chordata with a notochord which runs from 

 end to end of the body and lasts through- 

 out life, an atrium, and very numerous 

 gill-slits provided with tongue-bars ; with- 

 out definite brain, heart, limbs, or skeleton 

 of bone or cartilage, 

 e.g. Amphioxus. 

 fi. Subphylum Vertebrata. 



Chordata in which the notochord does not 

 reach the front of the head and is usually 

 reduced or lost in the adult, without atrium, 

 with few gill-slits, which are without tongue- 

 bars and are often lost in the adt»lt ; with 

 well-developed brain, heart, usually two 

 pairs of limbs, and always an internal 

 skeleton of bone or cartilage. 



a. Class Cyclostomata. 



Cold-blooded Vertebrata with persistent 

 gill-clefts, without any trace of jaws, 

 limbs, or scales, and with median nasal 

 opening. 



e.g. Petromyzon. 



b. Class Pisces. 



Cold-blooded Vertebrata with paired fins, 

 bony scales, rays in the median fins, 

 persistent gill-clefts, and no lungs, 

 amnion, or allantois. 

 i. Sub-class Elasmobranchii. 



Cartilaginous fisheswithout an air-bladder, 

 e.g. Scyllium, Raia. 



