THE EVIDENCE FROM DEGENERATION. 



357 



history. Thus it is matter of sober natural-history fact that a 

 sea-squirt larva, of all invertebrate animals, is the only being that 

 possesses organs and parts proper to the young vertebrate or to the 

 adult form of one lower vertebrate in particular. This adult is the 

 little fish known as the lancelet, which, in the relative simplicity of 

 its organisation, makes a nearer approach to the poor (or sea-squirt) 

 relations of the vertebrates than any other fish. 



The fact of vertebrate and sea-squirt relationship is worth dwell- 

 ing upon, because the topic unquestionably presents one with a 

 common point of view, whence rf 

 a survey of the higher develop- ^ 

 ment, evolution, and progress 

 of the vertebrates, and a view 

 of the degeneracy and retro- 

 gression of the sea-squirts, 

 may best be obtained. Re- 

 velling in the freedom of its 

 early life, the larval sea- 

 squirt presenting, as already 

 noted, a striking resemblance 

 to the tadpole of the frog, in 

 its backbone, its nerve-system, 

 and its breathing-sac, or modi- 

 fied throat ultimately settles 

 down. Like the youthful bar- 

 nacle somewhat, the young 

 sea-squirt attaches itself to a 

 stone or shell by the suckers 

 with which nature has pro- 

 vided its head. Then suc- 

 ceeds the disappearance of 

 the tail, with its backbone 

 and its nerve cord, and the 



body itself soon assumes the sac-like shape that betokens the 

 mature ascidian character. The outer skin becomes tough and 

 leathery, and develops the cellulose which, by biological right, we 

 might expect to find in plants alone. Then succeeds the fuller 

 formation of the gill-sac or breathing chamber, and of its neighbour 

 compartment, which receives the effete water of respiration to be 

 ejected by the second mouth of the sac-like body. The eye of the 

 larva likewise disappears, and all that remains to the adult ascidian 

 is a nerve mass, called by courtesy the " brain," and which serves to 

 regulate the few acts that mark the placid and rooted existence of 

 the race. 



Attention has been recently directed in a special manner to 



FIG. 257. DEVELOPMENT OF SEA-SQUIRT. 



