5 6 4 
SEMIVERTEBRA TES. 
ontorowins: 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 tilled 
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 
