26 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



canal is thicker-walled than the other part; I have not been able to make 

 out the meaning of this structure satisfactorily. Upon the whole, these 

 structures in the young Pentacrinoid would deserve to be very carefully 

 worked out on material preserved specially for histological study. On the 

 inner side of the parietal canal is seen a small body, which apparently rep- 

 resents the primary genital gland (comp. under Tropiotnetra, p. 16). It 

 may still be mentioned that the pharynx in the specimen figured was very 

 distinctly compressed. In another specimen sectioned this was, however, 

 not the case, so that it is not safe to state that this is a characteristic feature 

 of the Pentacrinoid. The axial organ was found to be only slightly developed. 



The formation of the skeleton is just beginning in the first of the embryonal 

 stages represented (plate xii, figure 3). As in Antedon and Tropiometra, the 

 oral and basal plates do not lie exactly opposite one another. At this stage 

 8 to 10 stalk-joints are present. In the figure quoted is seen a single very 

 small plate, just a small grain lying inside the circle of basalia. This is, as 

 becomes evident from a comparison with the following stage, the first rudi- 

 ment of the infrabasalia. 



In the next stage (plate xii, figure 4) the skeletal plates have enlarged 

 considerably and have the usual fenestrate structure. In this stage 4 plates 

 much smaller than these and with only one or a few holes are seen inside the 

 basalia. These are the infrabasalia. Generally there are 4 of them, but 

 sometimes only 3, or more rarely 2, and in no case were 5 found. They are 

 about equal in size, and there is no indication that one of them might be 

 double, so that the number would virtually be 5. About 18 stalk-joints are 

 present in this stage, when the embryo is ready to leave the egg-membrane 

 and swim out to find a place for attaching itself and transforming into 

 a Pentacrinoid. 



2. THE PENTACRINOIDS. 



The Pentacrinoids were found attached to Corallines growing on the 

 rocks in the same localities where the grown specimens were found. Although 

 they were not reared, there can be no doubt that they are really the Penta- 

 crinoids of Compsometra serrata. This was the only species of Crinoid found 

 in this locality; only once a single specimen of another species was found, 

 but this was a 20-armed form, a young example of Comanthus japonica (?), 

 which occurs abundantly in other localities near the biological station at 

 Misaki. That the Pentacrinoids can not belong to a 20-armed species is 

 proved by the later stages, which are 10-armed. One more species of Crinoid 

 is common in the same localities as the large Comanthus, viz, Tropiometra 

 macrodiscus.^'' But the Pentacrinoids can not belong to that species, as is 

 shown by the remarkable spicules of the Pentacrinoids, described below, 

 which do not occur in the Tropiometra, while they are identical in the 

 grown Compsometra. 



" I am indebted to Dr. A. H. Clark for these names. 



