THE DEVELOPMENT OF ASTERINA GIBBOSA. 395 
Synapta on which Sémon based his theory. In all Holothu- 
rids the buccal tentacles spring like the buccal tube-feet of 
Echinids from the proximal portion of the radial canals. It is, 
however, difficult for me to see how anyone can doubt that the 
Asterids are the least modified group of the Echinoderms, I 
have already dealt with their relations to Ophiurids, and have also 
pointed out that the Asterid central nervous system is really a 
concentration of the diffuse nervous plexus in connection with 
what must be regarded as a great sensory tentacle,—that, in fact, 
the whole radial water-vascular canal is to be regarded as a 
pinnately branched tentacle for which the arm is a secondary 
support. Sémon himself has suggested this (20), and it comes 
out even more clearly in Crinoid development than in the case 
of Asterids. Now the long radial canals in Echinids, ending 
in degenerate sense tentacles, clearly at one time had arms to 
support them; but these supports have been drawn back into 
the body. The Holothurids have been probably derived from 
the primitive Echinids; their calcareous nodules are most 
likely plates and spines atrophied in order to allow of free 
muscular movement. The terminal sense tentacles of the 
radial canals have entirely disappeared, and the forward shift 
of the madreporite and genital opening is no more difficult 
to understand than the varying position of the anus in Echi- 
nids. In the Asterids alone is locomotion entirely dependent 
on the tube-feet, and in them only we have the nervous system 
exposed. 
On the second question, viz. that of the affinities of the 
Echinodermata as a whole, much light is thrown by the 
development of Asterina gibbosa. It is of course well 
known that the Tornaria larva of Balanoglossus shows a strong 
resemblance to the Bipinnaria in the course of its ciliated 
bands, and in possessing a preoral celom opening by a pore on 
the left. The adult Balanoglossus has five celomic cavities, 
and Bateson has shown that these arise as separate pouches of 
the gut, The question arises whether it is legitimate to 
homologise with these the five coelomic cavities of the Asterina 
larva which arise by division of pouches already formed, but 
