14 STUDIES IN THE DEVELOPMENT OF CRINOIDS. 



others not till they are 7 or 8 days old — the processes which accompany 

 the transformation from the free-swimming larva to the young Penta- 

 crinoid may pass at very different speeds, and it is thus not possible to say 

 at which age one or another stage of the later development occurs. It 

 may be said only that it is the fixation that hastens the development, 

 while up to that time the developmental processes are going on very slowly 

 or not at all. 



As mentioned in the introduction, it often happened that embryos which 

 had attached themselves to unsuitable objects, especially the leaves of 

 Posidonia, dropped off and fell to the bottom of the jar. The reason for this 

 evidently was that Diatoms and other micro-organisms growing on the 

 Posidonia leaves hindered the fixation to the leaf. Doubtless the suctorial 

 disk secretes some kind of fluid by means of which the fixation takes place, 

 but in this case it would not act, because the disk could not touch the surface 

 of the leaf itself. 



The embryos thus lying on the bottom did not die; neither did the devel- 

 opmental processes cease; but the development went on abnormally, result- 

 ing in the embryos assuming a pecuhar pipe-like shape (plate ix, figure 6). 

 It is the vestibulum which is first affected by the failure of the fixation. The 

 process of closing does not continue till the end; the fold of the epidermis, 

 which normally gradually covers up the invagination, stops growing when it 

 has reached about midway (plate vii, figure 8). The aboral (previously 

 anterior) end of the invagination remains unaltered. When the vestibulum 

 closes normally a remarkable rotation takes place, as is well known through 

 the elaborate researches of Barrois and Seehger, the vestibulum wandering 

 from the ventral side to the oral (previously posterior) end of the larva, carry- 

 ing with it the hydrocoel and other internal structures. While at first the 

 vestibulum is contiguous with the disk of fixation, it now hes at the opposite 

 end of the larva. There has thus taken place an enormous prolongation of 

 the part of the epidermis lying between the disk of fixation and the vestibu- 

 lary invagination — that is, the part occupied by the first (in Antedon second) 

 vibratile band. The closure of the vestibulum is, so to say, the sign for the 

 starting of this prolongation. But when the closure is not complete the 

 prolongation of this part of the epidermis does not take place, there is no 

 stimulus to start it, and the vestibulum continues to be contiguous with the 

 disk. Meantime the developmental processes otherwise go on normally, 

 and the stalk-joints of the young Pentacrinoid especially grow rapidly; but 

 gradually there is no room for the stalk and it has thus nothing left but to 

 curve in an arch, and the embryo becomes humpbacked ! 



In spite of the incomplete closure of the vestibulum, the hydrocoel and 

 other internal structures continue their development normally, and as my 

 material of normal embryos in the fixation stage is very limited, I have found 

 it necessary also to use these pipe-shaped, humpbacked specimens for the 



