THE DEVELOPMENT OF ASTEEINA GIBBOSA. 279 



is the removal of the nervous system from the surface effected 

 by invagination leading to the formation of a neural canal ? 



When we come to try and picture the characters which the 

 Dipleurula possessed, we see at once that it must have been 

 far more primitive than any existing form. In point of fact 

 an Asterid is about the most undifferentiated animal above the 

 level of Coelenterates which exists. No proper blood-vessels, 

 no specialised excretory organ, a central nervous system which 

 is really a local concentration of a diffuse skin plexus, perfectly 

 simple generative ducts, a most feebly developed muscular sys- 

 tem, the fibres being for a considerable time simply myo-epi- 

 thelial cells, — where is such a state of things to be found outside 

 the Coelenterata ? When we further add that in the Crinoid the 

 ambulacral nervous system nearly atrophies in the adult, and 

 is replaced by a new system developed in a totally different 

 position, we see that we are at about as low a level as one could 

 well imagine, since the central nervous system in all higher 

 forms is a most persistent structure. 



Assuredly Platyhelminths, which have been usually regarded 

 as the basal group in the Coelomata, or better, Triploblastica, 

 are far more highly specialised. To say nothing of their 

 cephalic ganglia, we have their highly developed muscular 

 wall- and their complicated excretory and genital organs to 

 prove this. 



We shall not, then, go far astray in assigning the Dipleurula 

 and the Tornaria to a group, the Protocoelomata, which were 

 not far removed from the Coelenterates ; the ccelom was 

 divided into three parts on each side, but of these the most an- 

 terior were usually fused to form an unpaired vesicle. The 

 Dipleurula differed from the Tornaria chiefly in possession of an 

 aperture, the stone-canal, in the wall separating the proboscis 

 coelom from the collar coelom. This may have been the primi- 

 tive arrangement, or it may have been a secondary arrangement 

 acquired in consequence of the Dipleurula having lost the collar- 

 pores, one of which may, however, as we have seen, be developed 

 as a variation in the Asterid larva. At the apex of the prseoral 

 lobe was a more or less developed sense-organ with associated 



