42 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



but still perfectly recognizable remnants of larvae (plate xix, figure 5). I 

 have even found quite young Pentacrinoids with the vestibulum just opened 

 and arms not yet developed, with an embryo almost of their own size in their 

 mouths. On account of the large number of Pentacrinoids found attached 

 in clusters to the tip of the upturned cirri (Andersson has counted no less than 

 99 Pentacrinoids in one specimen), this danger to the embryos is very real, 

 and probably quite a large number of them must thus perish. 



K. A. Andersson has observed (op. cit., p. 6) in two cases a coalescence 

 between the larva and the wall of the marsupium, the coalescence having 

 taken place at the posterior end of the larva. He suggests that this may 

 possibly be an adaptation serving to the better nourishing of the embryo; 

 still he does not lay much stress on this point, as he agrees that it may be 

 quite an accidental case. There can be no doubt that it is an accidental 

 abnormality. The fact that the larvae remain within the egg-membrane until 

 they are ready to leave the marsupium is proof enough that there can be 

 no question of their coalescing with the marsupial wall with the object of 

 extracting nourishment from the tissues of the mother. If there has really 

 been such a coalescence in the cases observed by Andersson — ^and the figure 

 he gives (taf. ii, figure 10) certainly shows that — it must be due to a casual 

 rupture of the egg-membrane. 



Andersson further regards the embryo showing this coalescence as a 

 double monster. Judging from the figure, this is a mistake. Evidently 

 he has been misled by the strong development of the suctorial disk, which in 

 the figure quoted is nearly as large as the vestibulum. 



I have observed in one case an embryo within a marsupium having devel- 

 oped into the pipe-shape described above for Tropiometra. This must be due 

 to the larva not having succeeded in leaving the marsupium, and having 

 thus been unable to attach itself, it has developed in the same way as the 

 unattached Tropiometra larvae. 



The young, newly attached Pentacrinoid offers some interesting struc- 

 tural features. As seen from figure 1 of plate xxi, the vestibulum now has 

 assumed the normal place at the formerly posterior (now anterior) end, and 

 it has a distinct lumen. The tentacles have not yet become free. In this 

 stage the Pentacrinoid is otherwise a poor object for the study of the inner 

 structure; hardly anything except the stomach can be clearly distinguished 

 in decalcified specimens, stained and mounted in balsam. In sections (plate 

 xviii, figures 1, 2, and 7) a very conspicuous difference from the larva is 

 shown to exist in the histological character of the entoderm. A single, 

 fairly regular layer of nuclei is seen along its outer surface, while the whole 

 inner part of it is filled with a dense mass of small grains that stain very 

 strongly in hematoxylin. There is no doubt that this corresponds to the 

 mass of small cells that fills the lumen of the entoderm in the corresponding 

 stage of the young Antedon, while nothing quite corresponding was observed 



