TROPIOMETRA CARINATA. 15 



study of the development in the stage of transformation from the free- 

 swimming larva to the young Pentacrinoid. 



The fact that these humpbacked embryos may develop into true Penta- 

 crinoids differing from the normal ones only in having the head bent down- 

 wards, like a drooping flower, is of no small interest. Using the Posidonia 

 leaves for the larvae to attach themselves to has thus resulted in an unin- 

 tentional experiment to test the significance of this remarkable embryonal 

 structure, the vestibulum. 



Barrois (pp. 642-644, plate xxx, figures 21 to 24) has recorded a similar 

 unintentional experiment with the larvae of Antedon, the result being the 

 same, that the development of the internal organs goes on normally despite 

 the abnormal position. Only it does not appear that Barrois has seen them 

 develop into true Pentacrinoids. Seeliger (p. 266) also has seen abnormal 

 embryos developing their tentacles, although the closure of the vestibulum 

 had not taken place (even less so than in the cases observed by Barrois and 

 the present author), there being in the cases observed by Seeliger no covering 

 up at all of the vestibulary invagination, so that the tentacles are at once 

 free. Seeliger has not seen these abnormal embryos develop into Penta- 

 crinoids. As pointed out by Seeliger, the Crinoid embryo with tentacles 

 developed on the ventral side described and figured by Busch 14 must be such 

 an abnormal embryo. 



The main point, or at least the most conspicuous, in the transformation 

 of the embryo from free-swimming to fixed is the closure of the vestibulum. 

 This proceeds in the same way as in Antedon, so well known through the care- 

 ful researches of Barrois and Seeliger. An early stage of this process is seen 

 in plate vi, figures 2 and 3, from a specimen 3 days old; it corresponds well 

 with Barrois's diagrammatic figure, plate xxvi, figure 13, and Seeliger's taf. 

 19, figure 114. The completely closed vestibulum is seen in plate vn, figure 3, 

 representing a longitudinal section of an embryo 8 days old, just attached. 

 To enter on a more detailed description of the process seems unnecessary. 



The glandular character of the cells of the vestibulum is indicated by 

 the fact that they retain the hematoxylin as strongly as do the glandular 

 cells of the outer ectoderm, but it disappears with the closure of the vestibu- 

 lum (compare plate vn, figure 3, with plate vi, figure 2) . Also in the epidermis 

 itself the glandular cells disappear; in the stage plate vi, figures 1 to 4, they 

 are still fairly numerous, especially at the anterior end, where previously 

 the apical pit was; also at the posterior end they are still plainly visible. 

 In plate vi, figure 8, and plate vn, figure 5, only a few are still visible; while 



14 W. Busch. Beobachtungen uber Anatomie und Entwickelung einiger wirbellosen Seethiere, 1851, 

 p. 87, taf. xiv, figure 6. The embryos represented in his figures 3 and 5 are also abnormal, while the one 

 represented in figure 7 can not possibly be a Crinoid embryo at all, not even an abnormal one, on account 

 of the hooks developed in both ends, on which Busch lays so much stress, taking them to be the future 

 points of the arms. Such hooks do not occur in the Crinoid embryo. What this figure really represents 

 it is hardly possible to ascertain. The hooks recall those of the Ophiothrix embryo about to metamorphose, 

 but the figure would certainly be very fantastic for an Ophiothrix in metamorphosis. 



