A MONOGRAPH OF THE EXISTING CRINOIDS 311 



The coelomic vesicles have assumed their final position, the left at the oral end, 

 between the hydrocoel and the entoderm, the right at the aboral end of the entoderm, 

 and they are now distinguished as the oral and aboral coelom. The aboral coelom 

 has formed the vertical mesentery, and in it the axial organ has made its appearance. 

 Also the chambered organ is assuming its final shape and is seen very distinctly con- 

 tinuing through the whole length of the stalk; but this was already the case previous 

 to fixation. 



The entoderm undergoes very important changes. First the rectum develops 

 after the fixation, but it does not open externally there is not even an in vagina tion 

 of the epidermis to meet it. Then the mouth opens into the vestibulum. There is 

 an invagination of the thick ectodermal layer forming the bottom of the vestibulum 

 which meets the entoderm and forms the esophagus. 



A very conspicuous feature in the development of the intestinal tract in Antedon 

 is -the wandering of cells from the wall of the stomach into its lumen to be devoured 

 there and thus to provide nourishment for the young crinoid until its mouth opens and 

 it can procure its own food. This remarkable method of feeding itself is not seen 

 very clearly in Tropiometra. In fact, Mortensen was not at all sure that it occurs in 

 this genus. He says that in Antedon the object of these cells is certainly not to be 

 devoured by the larva, as assumed by Seeliger; instead he believes Bury to be right in 

 regarding them as phagocytes which produce an histolysis of the larval tissues, 

 especially of the entoderm. 



Some of the Mortensen's larvae attached themselves to the surface film of the 

 water, developing into very fine pentacrinoids, and the specimens used for sectioning 

 were from these. None of them reached so far as to open up the vestibulum, but the 

 covering of the vestibulum is thinning out in the middle and must be very near the 

 time of opening. The other pentacrinoids that went so far in their development 

 did not afford sufficient material for study by means of sections, but as none of them 

 went much farther in their development in none of them has the formation of the 

 arms begun there could hardly be expected any noticeable progress in the internal 

 development beyond the stage figured. 



The ectoderm has lost the last trace of its original character, and there is hence- 

 forth no distinguishing of the former ectoderm cells from the mesenchyme cells; 

 also, the glandular cells have completely disappeared. 



The basal disk, which could be readily studied intact in these specimens from the 

 surface film in which no force was needed to loosen the attachment, shows the interest- 

 ing feature that a rather thick caticular layer has been secreted. This probably 

 represents the secretion by means of which the attachment is effected. A similar 

 thick cuticula on the base of the stalk was also observed in Antedon by Seeliger. 

 The upper side of the basal disk contains numerous small grains which stain strongly 

 in hematoxylin, the cell nuclei among them appearing only very lightly colored. 



In the vestibulum the thinning out of the high epithelium of the basal wall has 

 just begun. The formation of the oral nervous system apparently has not yet begun. 



The hydrocoel ring has just closed; in the anal interradius a narrowing of the 

 lumen of the ring is still seen, across which a thin dissepiment runs. This is no 

 trabecule, but the joined end walls of the two ends of the hydrocoel. It is still intact, 

 so that the lumen of the ring is not yet continuous. The epithelial lining of the 



