58 ZOOLOGY 



vertebra, which literally signifies something turned in a lathe, 

 is applied to the disc-like segments of which the backbone of 

 the higher Vertebrata is made up, but in all Vertebrata in the 

 early stages of development, and in the simpler forms through- 

 out life, the " backbone " consists of a gelatinous string termed 

 the notochord (lit. back-string). The vertebrae are formed 

 later round this string, and it is then more or less absorbed. 

 The hollow nerve-tube has in the vast majority of cases a great 

 hollow expansion at its front end termed the brain. In the 

 higher Vertebrata the walls of this expansion become very 

 thick and its cavity small. The Vertebrata include ourselves 

 and the animals which most resemble us, viz. the warm-blooded 

 hairy quadrupeds or mammals; but they also include the 

 birds, reptiles, amphibians, and fishes, and moreover a group 

 of forms which the older naturalists classed as Invertebrata, 

 viz. the sea-squirts or Ascidians. These last are most 

 wonderful forms. When young they lead an active life, swim- 

 ming gaily in the sea, and in this stage of their existence they 

 possess a notochord, a hollow brain, a tubular nerve-cord, and 

 a pair of gill-slits. They soon, however, attach themselves to 

 a rock and undergo rapid degeneration. The nerve-cord and 

 notochord disappear, the brain becomes a small solid ganglion, 

 but the single pair of gill-slits become enormously enlarged and 

 converted into a trellis- work by the growth of partitions across 

 them. The bars forming these partitions carry cilia, which 

 serve to drive the water out which passes inwards by the 

 mouth. The animal in fact converts itself into a vast filtering 

 apparatus just as does a Sponge, only that the current is in 

 the opposite direction. The Sea-squirt becomes enveloped in a 

 gelatinous external skeleton, exuded by the skin and containing 

 a substance akin to paper, and no one looking at a Sea-squirt 

 adhering to a rock, and much resembling in outer appearance a 

 Sponge, would imagine that it had any affinity whatever with 

 a Vertebrate (Fig. 27). 



The more normal Vertebrata which retain the backbone 

 throughout life are divided, as we have seen, into Fishes, 

 Amphibians, Reptiles, Birds, and Mammals. These five 

 groups have, however, bonds which unite some of them more 

 closely with one another than with the rest. Thus Fish are 

 separated from the other four groups by the circumstance 



