J22 



ZOOLOGY 



Tunicates 



of early Palaeozoic times, and these Eurypterids he 

 thinks may be the ancestors of the curious extinct 

 creatures called "Ostrocoderms," which seem to lead 

 toward the true vertebrates. However this may be, 

 no one sees in the living Prochordates the actual types 

 which gave rise to the vertebrates, but only animals 

 possessing some of the characters which those ancestors 

 must have possessed. They show us, in some measure, 

 how the evolution may have taken place, and represent 

 the unprogressive remnants of a group, most of which 

 either died out entirely or evolved to higher things. 

 They are therefore far more interesting than their 

 undistinguished superficial appearance would suggest. 



3. The Tunicata or Ascidians are marine animals 

 which in the adult state appear under a variety of 

 forms, some attached to rocks, others floating in the 

 open sea. The name "tunicate" is derived from the 

 tunic or coat forming the outer layer of the animal. 

 The commoner species, known as "sea squirts," are 

 found attached to rocks ; when irritated they rapidly 



Drawings by W. P. Hay and R. Weber 



FIG. 115. i, lateral view of an ascidian, and 2, a diagram of its anatomy, a, incur- 

 rent orifice ; b, excurrent orifice ; c, branchial basket ; d, stomach ; e, nervous system. 

 3, another species of ascidian (Sty da). 



