vii ANNELIDA 167 



ated ventral nerve-cord. The brain is, of course, to be compared to 

 the ganglion cells underlying the apical sense-organ in Ctenophora, 

 which have been experimentally proved to act as a co-ordinating 

 centre for the ciliary activity of the ribs. The ventral nerve-cord 

 originates as two longitudinal thickenings of the ectoderm, situated 

 at the sides of the mid-ventral line. 



Now in many larvae in the post-trochophoral stage (of. Echiuriis, 

 Oapitella), the mid- ventral line is occupied by a ciliated groove. This 

 groove extends from the metatroch behind the mouth to the telotroch 

 just in front of the anus, and thus it corresponds roughly to the 

 portion of the blastopore which closes in the process of separation 

 of primitive mouth from primitive anus. This process in the history 

 of the race must have been an extremely gradual one, and while the 

 undivided opening was in the figure-of-eight stage the whole circuit 

 of its lip was probably fringed with cilia, whose activity would assist 

 in the seizing of food. Just, then, as there is a ring-nerve underlying 

 the prototroch, so we might expect to find nerve fibres underlying 

 this ciliated border, and out of these nervous strands we may suppose 

 the ventral nerve-cord to have been built up. If such a circurn-oral 

 nerve existed in the original creeping Ctenophore it would certainly 

 l)e connected with the nerves radiating from the apical centre, and 

 one pair of these connections may have persisted as the nerve collar. 



The nephridia here essentially similar to those of Nemertinea 

 and Platybelminthes are a new acquisition. The original function 

 of getting rid of excreta would be naturally concentrated in the 

 ectoderm. When, owing to increase in size and muscular activity, 

 the excretory surface of the ectoderm became insufficient for this 

 purpose, ingrowths and in foldings would take place which would 

 increase its surface and its efficiency, and these ingrowths we suppose 

 to have given rise to the primitive nephridia ; and their embryonic 

 history bears out this view. 



It would be an extremely interesting thing to investigate the 

 excretory processes in Ctenophora, for thus it is possible that the first 

 fore-shadowings of primitive nephridia might be discovered. But by 

 no means all the excretory work in a Coelenterate is performed by the 

 ectoderm, since the endodermal cells also get rid of some excreta into 

 the gut cavity, whence it is ejected to the exterior. This function would 

 still be carried on by the lateral pockets of the gut, when they were 

 separated from the main axial portion as coelom. Thus the coelomic 

 lluid would tend to become charged with excreta, and so it would 

 have to be periodically got rid of. This would be facilitated by the 

 formation of coelomic pores, such as actually occur in the earthworm, 

 and it seems to have also been accomplished by the fusion of the 

 coelomic wall with the tubes of the ectodermal nephridia, a process 

 which Groodrich believes to have given rise in some cases to the 

 internal funnels of these nephridia, when they occur. In many cases 

 also the genital duct, which originated as a coelomic pore, is utilized 

 for this purpose, and in this way the larger trumpet -shaped 



