THE SEA SQUIRTS OR ASCIDIANS 



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in its investing membrane, the embryo assumes a ventral curvature, and at the 

 same time produces a long tapering tail, which eventually becomes coiled round it. 

 In addition to certain other structures, this 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 lance- 

 let, makes its appearance; and likewise an unpaired eye, agreeing precisely in 



A CARTILAGINOUS FIXED SEA SQUIRT, Phdllusid. 

 (Natural size.) 



structure and mode of development with the rudimental median eye of the tuatera. 

 After certain other changes, among which the development of a stomach and intes- 

 tine are included, the larva is ready to burst from its membranes, which it does by 

 spasmodic 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 



