564 SEMIVERTEBRATES. 



outgrowing tail includes the nerve-tube and the notochord; and in some forms 

 contains the only muscles developed at all. Subsequently a rudimentary brain, 

 corresponding to a simple structure in the lancelet, makes its appearance; and 

 likewise, an unpaired eye, agreeing precisely in structure and mode of develop- 

 ment with the rudimental median eye of the tuatera. After certain other 

 changes, among which the development of a stomach and intestine are included, 

 the. larva is ready to burst from its membranes, which it does by spasmodic 





A CARTILAGINOUS FIXED SEA-SQUIRT, Phallusia (nat. size). 



jerkings of the tail; and it thereupon starts on a free-swimming existence. 

 Before long the cellular structure of the notochord in the tail begins to disappear 

 by the formation of vacuities ; and eventually the whole structure becomes filled 

 with gelatinous matter. After a brief free existence it fixes itself by its muzzle to 

 some submarine object, with the tail stretched out and generally motionless. In 

 a short time this appendage commences to shorten, and finally disappears, by being 

 drawn up into the body of the developing sea-squirt and absorbed. A further 

 process of development results in the production of the perfect sea-squirt ; but it 



