Nov., 1918] Structure of A gelacrinites and Streptaster 79 



radial plates in these small specimens. Since, as I have shown 

 above, there is a possibility of a fusion of cover plates to form 

 the peristomial plates of the adult, there may not yet be rep- 

 resented more than enough plates to make these and the 

 differentiation into radial and inter-radial plates may first 

 occur at a larger size. 



By the time an individual developed its second (adult) 

 floor plate in each arm, the one outside of the five bounding the 

 substomial chamber, it should have two or three pairs of small 

 cover plates to indicate each arm on the oral side and the inter- 

 radial plates should be distinguishable. Such an animal should 

 be 2.5 mm. in diameter and none of this size were found with 

 the oral side exposed. 



Alimentary Canal. 



These very young specimens from the aboral side show 

 plainly what I take to be the path of the alimentary canal. 

 The skeletal plates are delicate, the animal a rapidly growing 

 one and it would be more likely that an animal killed with the 

 intestine dilated with food would have the position of the 

 skeleton modified by this intestine than that of a similar full 

 intestine would change the shape of the skeleton of an adult. 



Attention was called to this matter by finding numerous 

 cases of a seeming break in the skeletons of these small creatures 

 as seen from the aboral side. There is a depression extending 

 around the substomial chamber and just within the peripheral 

 ring. This depression begins at the posterior part of the 

 substomial chamber and is continuous with its cavity just at 

 the point where the aboral part of plate P will be in the adult. 

 It extends around just outside the first (and only) floor plates 

 from the anal inter-radial space back to this same interspace 

 again. The alimentary canal would occupy just such a position 

 if it passed posteriorly out of the substomial chamber and made 

 a complete circle around in the body cavity between the peri- 

 pheral rim and the ring of the first floor plates, to connect 

 with the anal pyramid in the posterior inter-space. Plate VI, 

 Figures 32 and 34 are photographs of two specimens showing 

 what I have just attempted to describe. There is no way of 

 deciding in these specimens whether the alimentary canal, after 

 leaving the substomial chamber posteriorly, twisted to the right 

 or to the left. Both photographs show the depression, con- 



