MEMOIRS. 



On the Oral and Apical Systems of the Echixoderms. 

 By P. Herbert Carpenter, M.A., Assistant Master at 

 Eton College. (Part I.) 



The following essay is an attempt to compare and criticise 

 the various views that have been put forward from time to 

 time^ respecting the homologies among the different Echino- 

 derms of the two groups of calcareous plates, which appear 

 at a very early period around the two peritoneal diverticula 

 of the primitive digestive sac of the larva. 



To the one group, developed upon the abactinal surface 

 of the body, around the right peritoneal sac from which the 

 aboral division of the coelom is derived, the name of the 

 Abactinal, Apical, or Dorsocentral System has been applied 

 by the various authors who have studied it. It generally 

 consists of three sets of plates, viz., a single central plate, 

 surrounded by two rings of five plates each. The plates 

 forming the proximal ring are situated interradiaUy , as 

 regards the general symmetry of the Echinoderm type, while 

 the plates of the distal ring alternate with them, and are there- 

 fore rac7«a7 in position. This compound Apical system forms 

 an essential constituent of the skeleton of all the Urchins, 

 Starfishes, and Crinoids, although the special functions of 

 its various elements differ very considerably in these different 

 groups. 



The other group of plates, which I will call the Oral 

 System, is developed upon the actinal surface of the young 

 Echinoderm, around the left peritoneal sac that gives rise 

 to the anterior or oral division of the coelom. It has hitherto 

 received but little attention from the various naturalists who 

 have studied the comparative morphology of the Echino- 

 derms. It is much simpler in character than the Apical 

 system, as in recent Echinoderms it consists of but one ring 

 of five plates disposed interradiaUy around the mouth of the 



VOL. XVm. NEW S£R, A A 



