58 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



not constricted in the middle. The upper 5 joints are gradually widening, 

 the upper one, carrying the cirri, being the widest. A considerable variation, 

 however, occurs in the shape of the stalk, both in the number of stalk-joints 

 and in regard to the shape, especially of the lower joints. Thus, in the stalk 

 figured in plate xxviii, figure 8, there are only 8 joints besides a ninth (the 

 centrodorsal) which has gone with the detached young Crinoid. Plate 

 XXVIII, figure 7, shows the 3 lower joints of another fully developed Penta- 

 crinoid; here the lower joint is small and wedge-shaped. In the Penta- 

 crinoid figured in plate xxviii, figure 5, the lower joint is also somewhat 

 wedge-shaped, but evidently would have become considerably larger than 

 the following joints, and in figure 3 the two lower joints would evidently 

 have become larger than the following joints, while in figure 4 the lower 

 joint is much smaller than the second. These differences have something 

 to do with the place to which the Pentacrinoid has attached itself, as a com- 

 parison of the figures quoted will show; the wedge-shape of the lower joint 

 (figure 7) is due to the fact that it is attached in the narrow space between 

 two pinnule joints, while the simple shape of the lower joint in figures 6 and 

 8 is due to the fact that it is attached to a simple surface either on the middle 

 or at the end of the pinnule joint. 



The most interesting feature in this Pentacrinoid is, of course, the total 

 absence of the terminal stem-plate, a fact otherwise unknown in Crinoids. 

 The reason for this would seem to be their habit of attaching themselves 

 within the marsupium. In remarkable contrast to this stands Notocrinus 

 virilis, with its supplementary terminal stem-plates. Both cases would seem 

 to be in some w^ay abnormal and due only to the special biological conditions 

 of these Pentacrinoids. I should not attach any special importance to the 

 morphology and homologies of the terminal stem-plate of Crinoids in these 

 two cases. But it is, of course, too early to draw definite conclusions regard- 

 ing this question from the few observations that have as yet been made 

 upon the development of Crinoids. It is a problem for the future. 



