ON THE OCCURRENCE AMONG ECHINODERMS 



OF LARVAE WITH CILIA ARRANGED 



IN TRANSVERSE RINGS, WITH A 



SUGGESTION AS TO THEIR 



SIGNIFICANCE. 



CASWELL GRAVE. 



In this paper a short account is given of some observations 

 made at the laboratories of the United States Fish Commission 

 at Woods Hole and Beaufort on the larvae of various echino- 

 derms. The attempt is also made to show that these observa- 

 tions, taken together with those made by other students of the 

 group, have a direct bearing upon one phase of the problem of 

 the early ancestry of the echinoderms. 



It would be quite impossible to give an intelligible discussion 

 of the bearing these observations are interpreted to have upon this 

 subject without first recalling the hypotheses which have been 

 put forward by other students of the group to account for its 

 origin and present organization. 



The hypothesis which now seems to have the most general 

 acceptance is not the work of any one mind but represents the 

 work of many. It would be difficult, therefore, in giving a hasty 

 review of its most important points, to credit each of its authors 

 with just his contribution, so I shall make only such comments in 

 passing as will serve to explain the changes and additions which 

 seem to me to be warranted. 



OBSERVATIONS. 



Holothurians. 



The barrel-shaped pupae of Holothurians have been long 

 known, having been described by Muller, 1 Semon 2 and others. 

 They arise in each case by the breaking up and rearrangement of 



1 J. Miiller, " Abhandlungen iiber die Larven und Metamorphose der Echinoder- 

 men," Abl. Kgl. Akad. Wiss. Berlin. 



2 R. Semon, " Die Entwicklung der Synapta digitata, und die Stammesgeschichte 

 der Echinodermen," Jena Zeitschr., Vol. XXII., 1888. 



169 



