THE DEVELOPMENT OF ASTERINA GIBBOSA. 397 
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 Ceelomata, or better, Triptoblastica, 
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 Protoccelomata, which were 
not far removed from the Celenterates ; the colom 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 
ccelom from the collar ccelom. 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 preoral 
lobe was a more or less developed sense-organ with associated 
