190 



THE DARWINIAN THEORY 



is represented merely by a single ganglion, and there 

 are no sense organs. The notochord is absent. In 



Clavelina, a compound A scidian , showing the anatomy of an adult member 

 of the colony-, and two members in early stages of development ( produced 

 asexually). The members of the colony are connected togetker by the stolon, 

 which consists of an outer tube of 'ectoderm continuous with the outer layer 

 of the individuals ; and an inner tube of ' endoderm, which is a direct pro- 

 longation of the so-called " epicardium." 



a, Anus ; ad, Dorsal nerve cord ; at, Atrial aperture ; br, Branchial aper- 

 ture ; cl, Cloaca ; dl. Dorsal languets ; e, Endostyle ; ep, " Epicardium " ; 

 g, Genital ducts ; h, Heart / i. Intestine ; k, Sub-neural gland ; ,1, Nerve 

 ganglion ; 0, Ovary ; oe, (Esophagus ; p, Pharynx, with numerous gill- 

 slits ; s, Stomach ; st, Stolon ; t, Testis. 



(Clavelina is more primitive than most Ascidians in retaining the dorsal 

 nerve cord in the adult condition.) 



fact, there is nothing in the adult Ascidian to indicate 

 Vertebrate affinities. (Fig. 35.) 



