312 Proceedings of the Royal Physical Society. 



it fixes itself, and development then proceeds spirally from 

 the antero- ventral part to the postero-dorsal. 



7. The perihsemal cavities arise very much as described by 

 Macbride in Asterina. 



8. Certain of the young star-fishes rotate their preoral lobe 

 through 90° or so, in such a way that the water- vascular 

 ring is then ventral if the larval plane be judged by the 

 arms of the lobe, as described by Goto, but this does not 

 appear to have the important significance attached to it by 

 that author. There is no question that the left and right 

 of the larva become the ventral and dorsal sides of the 

 adult. 



In giving these leading points, we may note that the two 

 enterocoeles arise in this species as in Antedon, and in this 

 respect differ from all other Asteroidea. It follows that there 

 is no difficulty in determining the subsequent fate of each. 

 Again, the right hydroccele appears to be present only as 

 long as the piano-symmetry (bilateral) of the larva is main- 

 tained, whilst the preoral sac remains throughout life as a 

 (? contractile) vesicle, which is evidently homologous with 

 the pericardium of Balanoglossus and Cephalodiscus, so that 

 an even closer comparison with these can be instituted. I 

 hope to show that the much maligned vascular system of 

 the star-fishes is really existent and comparable to that of 

 Hemichorda. 



Certain conclusions with regard to the general morphology 

 of the Echinoderms can be merely stated here. 



There appears to be evidence that the dipleurula stage 

 (with two hydrocoeles and a preoral sac and five pairs of 

 tentacles) first took to the habit of lying on its right side, 

 much after the manner of the Pleuronectids, and that in 

 consequence of this the left side was hypertrophied at the 

 expense of the right, the mouth moved to the left (upwards), 

 and the five left tentacles tended to surround the mouth. 

 After this, and independently of it, the animal further be- 

 came attached by the preoral lobe, when the well-known 

 movement through 180° of the principal organs, and the 

 adoption of axial symmetry, resulted (as in Antedon). Only 

 after this was effected did the Asteroids lose connection with 



