90 MR. P. H. CARPENTER ON THE GENUS ACTINOMETRA. 



closed within the radial pentagon, and from the centrodorsal coeloni, by the small 

 bony bars at their expanded central ends, which form the thickened inner or central 

 edges of the dorsal faces of the first radials (PL V. fig. 9 b, c.) They are slightly 

 developed in one of the varieties of Act. polymorpha, in which the radial areas of the 

 small centrodorsal piece (PI. VI. fig. 17, r.ar) and the corresponding dorsal faces of the 

 first radials (PI. VII. fig. I a, d) both exhibit median depressions, which gradually 

 increase in depth from their peripheral to their central ends. In both these cases the 

 centrodorsal piece is relatively very small and by no means conceals the first radials, 

 while its ventral surface rises very considerably from the circumference towards the 

 centre (PI. V. fig. 7, and PI. VI. fig. 17). The meaning of these radial spaces is to me 

 quite obscure. [n no other Comatula have I found any thing at all comparable to them 

 except in Act. robusta, where the axial radial canals give off horizontal diverticula (PL V. 

 fig. 10, r.c'), which extend outwards in a peripheral direction in the substance of each 

 first radial just beneath its dorsal surface. These, enclosing diverticula of the dorsal 

 portion of the ccelom, are, of course, in indirect communication with the external 

 medium, while the radial spaces in Act. pectinata are altogether outside the substance of 

 the first radials, communicate directly with the exterior, and are completely shut oil' from 

 the dorsal ccelom. There is, therefore, scarcely any resemblance between the two sets of 

 cavities, although they occupy very nearly similar positions, i. e. between the centrodorsal 

 piece and the whole or the greater part of the mass of the first radials. 



In some species of the fossil Apiocrmus, however, cavities similar to the radial spaces 

 in Act. pectinata appear to exist between every pair of contiguous basals and the first 

 radial, which rests upon them and alternates with them in position. As the basal circlet 

 is generally regarded as comparable to a stem-segment, it is evident that the posi- 

 tions of these cavities in Act. pectinata and in Apiocrmus respectively are homologous 

 with one another. In Apiocrinus rotundas five lateral openings were discovered by 

 Miller ' on the circumference of the body, " in or between the lateral surfaces of the 

 joints of the pelvis (basals) and the insertion of the first costal (radial) joints," which in 

 one case he thought he was able to trace as a canal or perforation " passing through the 

 joint of the pelvis into the space between it and the costal joints, extending perhaps 

 thence into the perivisceral cavity" (i.e. into the dorsal division of the body-cavity). 



Miller supposed these to be the openings of oviducts leading to an ovary situated in 

 this dorsal ccelom, just in the same manner as the five openings on the ventral surface 

 of the centrodorsal piece of Glenotremitcs were (till lately) regarded as genital openings, 

 although the genital glands of all the recent Crinoids with which we are acquainted are 

 situated in the arms and pinnules. 



Similar openings to those seen by Miller in Ap. rotundas have been described in Ap. 

 obconicus by Goldfuss 2 , who also supposed them to lead into the body-cavity. This is, 

 however, certainly not the case with the homologous openings in Act. pectinata. 



The interarticular pores in the upper part of the stem of Peiitacrinus are also homolo- 

 gous with the external openings of the radial spaces in Act. pectinata. They are the 



1 Op. tit. p. 31. 2 Petref. German, p. Is7, Taf. vii. fig. 5, a, b, c. 



