A MONOGRAPH OF THE EXISTING CRINOIDS 



49 



radial ossicle). On the other hand, Thalassocrinus and Hyocrinus have low disks. 

 The position of Gephyrocrinus is intermediate. 



Gisl6n concluded that a voluminous disk is of very common occurrence among 

 the primitively organized Articulata. Sometimes this high disk is combined with loss 

 of the proximal pinnules. In the comatulids, according to him, there is evidence that 

 defective pinnulation was once present in the occurrence of a proximal pinnule gap 

 during the juvenile stages. 



According to Gisle"n the genital glands of the pinnules are to be considered as 

 originally processes from the primitive gonad, to which the present axial gland corre- 

 sponds, losing their connection with it only at a later stage. There is no definite 

 correlation between the pinnule and the gonad, as has been assumed, for in some 

 recent forms, as Meiacrinus, Notocrinus, and Comatula, the gonads do not appear in 

 the pinnules, but in the arms. All the ontogenetic indications point to a late migra- 

 tion of the gonads into the arms and pinnules. We may therefore suppose, according 

 to Gisle"n, that the ventral sack originally enclosed, besides a part of the intestine, at 

 least a part of the gonad. When the ventral sack later absorbed the whole of the 

 disk, we may assume that the gonad extended under all the interadii. The perisome 

 then rose between the arms and caused, at least in a number of forms, a reduction of 

 the proximal pinnules. When the disk once more dwindled in size the migration of 

 the gonads into the arms began. In a number of forms there still remains, as a re- 

 minder of the high disk and pinnule reduction, a proximal pinnule gap in the adults, 

 best marked in the Bathycrinidae and Atelecrinidae, and appearing during ontogeny 

 in all comatulids. 



Gisle"n said it is noticeable that the Flexibilia, which never developed pinnules 

 and had no ventral sack, always had a perisome extending high up between the arms. 



Gisln remarked there seemed to him to be a certain amount of interest in trying 

 to ascertain the total volume of the gonads in some of the comatulids. He therefore 

 detached all the gonads from an arm in some specimens with greatly swollen genital 

 pinnules, weighed the mass thus obtained, multiplied this by the number of arms, and 

 divided the result by the specific gravity (1.1), and in this way obtained the total 

 volume of the gonad. From this he calculated the height to which the disk would be 

 raised were the gonads included hi it, obtaining the following results: Heliometra 

 glacialis (with the genital pinnules relatively inconspicuously distended), weight of 

 gonads, 7.43 gr. ; volume, 6.76 cc. ; radius of the disk 10.5 mm.; raising of disk by 

 inclusion of the gonads, 19.5 mm., corresponding to 14 brachials. As the disk reached 

 to the tenth brachial, a disk with the gonads included within it would reach to about 

 the twenty-fourth brachial. Antedon petasus; weight of gonads, 0.53 gr. ; volume, 

 0.48 cc.; radius of the disk, 3.3 mm.; raising of disk by inclusion of the gonads, 14.1 

 mm., corresponding to 18 brachials. As the disk with the arms folded reaches to the 

 seventh brachial, a disk with the gonads included would reach to about the twenty- 

 fifth brachial. Compsometra serrata; weight of gonads 0.36 gr. ; volume, 0.33 cc. ; 

 radius of the disk, 3.4 mm.; raising of the disk by inclusion of the gonads, 9.1 mm., 

 corresponding to 16-17 brachials. As the disk reaches to the sixth brachial, it would 

 reach, with the gonads included, up to about the twenty-second brachial. Gisl6n 

 said that it seemed as if the enlargement of the disk under the conditions given above 

 was rather considerable and fully sufficient to account for the reduction of the proximal 





