Nov., 1918] Structure oj Agelacrinites and Streptaster 77 



In Agelacrifiites the anal pyramid, which seems to be com- 

 posed of either a single ring of triangular plates or this ring 

 under-laid by a supplementary series of plates (a total of from 

 6 to 20 plates then, depending on the species) must have been 

 part of a similar respiratory device. Whatever oxygenation 

 might have taken place along the food groove, there certainly 

 could have been a large gaseous interchange from the rectal 

 water content through its walls to the coelomic fluid and thus to 

 all structures in the body. Figure 2, Plate I, Figures 15, 18 

 and 19 of Plate III, Figure 29, Plate V, Figures 22 and 23 of 

 Plate IV, Figure 33 of Plate V, show the plates of this pyramid. 

 Figure 41 of Plate VII, and Figure 52 of Plate IX, are diagrams 

 of the pyramid as seen from the aboral side. 



The starfish is said to have plates in the walls of the stone 

 canal and it is possible that these plates may be related in a 

 similar way to the mechanism for keeping the ambulacral 

 system of the starfish full of water. 



Water System. 



Not much information on this subject is furnished by the 

 specimens at hand. In one specimen from the Dyer collection, 

 Figure 2, Plate I, there are two plates in the ink circle in the 

 posterior inter-radial space by the proximal end of arm 5 which 

 appear as a duct had passed between them. 



A specimen marked A. pileus in the Geological Museum of 

 the Ohio State University shows a plate in this same region 

 which has a distinct pit or pore in it. 



Another specimen of A. pileus seen has this same plate, 

 but instead of a pore there is a notch on the edge of the plate 

 next the inter-radial space. 



According to Bather this is the location of the hydropore, or 

 one of the hydropores, which correspond to the madreporic 

 body in the starfish. He suggests that there may be as many 

 as five hydropores present, but I have been able to find traces 

 of this one only. 



Two specimens showing the aboral side from the Elkhorn 

 demonstrate corresponding pits next to ray 5 on the left side of 

 the substomial chamber in the inter-radial area which contains 

 the anal pyramid. The pit gives little evidence as to the 

 direction the water tube took. From the presence of the cir- 

 cular openings between the cover plates on each side of a ray 



