86 



SUB-KINGDOM (TYPE) VI. VEETEBEATA 

 (GHORDATA). 



THE body in the Vertebrata is usually composed of a number 

 of more or less definite segments (very obscure in the Cyclo- 

 stomatous Fishes), which are arranged along a longitudinal 

 axis. The nervous system is in its main masses dorsal, and 

 the neural and hsemal regions of the body are always com- 

 pletely separated from one another. The cerebro - spinal 

 nervous axis is underlaid by the structure known as the 

 " notochord," which, in adult life, is usually more or less 

 completely replaced by the bony axis known as the " spine " 

 or " vertebral column." The limbs are sometimes absent ; 

 but, when present, they are never more than four in num- 

 ber, and are always turned away from the neural aspect of 

 the body. 



DIVISION A. ICHTHYOPSIDA. 

 CLASS I. PISCES (FISHES). 



ORDER I. PHARYNGOBRANCHII. 



Fam. Cirrostomi. Amphioxus (Lancelot, fig. 78). 



(J. Muller, Ueber den Ban und die Lebenserscheinungen far Branehiostoma 

 htbricum, Abhandl. der Berl. Akad., 1842 ; Kowalevsky, Entwickelungs- 

 geschichte von Amphioxus lanceolatus, St Petersburg, 1867.) 



m 



Fig. 78. Pharyngobranclm. The Lancelet (Amphioxus lanceolatus), enlarged. 



