THE DEVELOPMENT OF ASTEEINA GIJBBOSA. 277 



Synapta on which Semon based his theory. In all other Holo- 

 thurids the buccal tentacles spring like the buccal tube-feet of 

 Echinids from the proximal portion of the radial canals. It isj 

 howeverj 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 

 3)ointed 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. Semon 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 piaeoral coslom opening by a pore on 

 the left. The adult Balanoglossus has five ccelomic 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 



