THE SKVLLBSS VERTEBRATES. 141 



of tlieir youth, and go downward and backward, becoming 

 scarcely higher than clams. But without doubt some fa- 

 vored forms, now lost to science, pressed onward and up- 

 ward, held on to their tails, preserved their notocord, be- 

 came active swimmers, dashed vigorously about after other 



FiQ. 148 —Diagram of an ascidian tadpole-like larva, p, suckers (?); s, incur- 

 rent, s', exeurrent, openings; «, nervous cord; ii, notocord: g, respiratory sac; 

 n', sense-cavity containing the eye (o') and the ear (o). (Magnified.) 



minute animals, as the lancelet does, until, in their de- 

 scendants, the notocord supported the entire body, and an 

 incipient brain was formed, approaching that of the lance- 

 let, imperfect as it is, and behold the backboned type of 

 life was established ! 



In the Balanoglossus worm, with its gill-slits in its 

 pharynx, and the central nervous 

 system situated above a notocord, 

 we have a sign-post among worm- 

 like animals pointing out the way 

 to the ascidians, the forerunners of 

 the vertebrates. 



In the highest molluscs, such as 

 the cuttle-fish, we have anticipa- 

 tions of the vertebrates in the prin- ^"^ wg.-Young Balanoglossus. 

 cipal nerve-centres being gathered into the head and there 

 forming a large brain, which is partially protected by pieces 

 of gristle, forming a slight brain-case; their eyes, moreover, 

 are nearly as perfect as those of a fish. But the cuttle was. 



