ISOMETRA VIVIPARA. 37 



third and fourth vibratile bands (plate xv, figure 9). Also in the section 

 represented in plate xv, figure 5, the hydropore is present, while the pore- 

 canal is not narrowed. However, it is not constantly opening outwards; 

 in some embryos of this stage there was certainly no hydropore (plate xv, figure 

 4) ; it may be emphasized that the lack of a hydropore in a case like that in 

 the figure quoted is a real fact, not a feature due to preservation. In the 

 embryo figured in plate xv, figure 11, there also seems to be no outer open- 

 ing, but there it may possibly be due to preservation. Of course, it may be 

 that the hydropore would have developed also in the embryos, where it is 

 not found in this stage. This I would be inclined to expect, and that the 

 matter is only one of variability in the time of appearance of this structure, 

 as there is unquestionably some variation in the time of appearance of 

 some of the developmental processes e. g., the formation of the vestibulary 

 invaginatiori or the formation of entoderm and ectoderm (see p. 32-33 ; plate 

 xiv, figures 4 and 5). 



The enteroccelic vesicles have assumed their final places, the left (the 

 future oral ccelom) at the posterior end, the right (the future aboral ccelom) 

 at the dorsal and anterior side of the entoderm, forming a mesentery where 

 they meet one another (plate xv, figure 6) . From the right enterocoel vesicle 

 or aboral ccelom prolongations into the anterior part of the embryo are 

 developing (plate xv, figure 10) ; they are the rudiments of the chambered 

 organ. That there are 5 of them, as in Antedon, can hardly be doubted, 

 although I have been unable to determine this in the sections at this stage. 

 The epithelial lining of the enteroccel vesicles has for the most part assumed 

 an endothelial character. 



In the mesenchyme are often found globular masses of yolk spherules of 

 different sizes, sometimes very large, as the one lying at the dorsal side in 

 the section figured in plate xv, figure 8. More rarely such yolk globules 

 may be found also in the ectoderm (plate xv, figure 4), while in the entoderm 

 the yolk spherules are generally not united into distinctly limited globules. 



4. THE FULLY FORMED LARVA. 



The shape of the fully formed larva is markedly different from that 

 of the typical Crinoid larva (plate xxu, figures 1 to 8). It is flattened on 

 the ventral side and has a distinct constriction in the middle of the body 

 (see especially figures 6 and 7), between the two circles of skeletal plates, 

 the oral and the basal plates. The vestibulary invagination has the shape 

 of a very narrow slit (in plate xxu, figure 2, it is broader than usual, on 

 account of the more contracted condition of this specimen). The suctorial 

 disk is very distinct, often with thickened edges, the anterior end being then 

 almost snout-like and prominent (plate xxu, figure 3). The state of con- 

 traction accounts for the different shape of the anterior end of the larvae, 

 as exemplified in figures 2 and 3. The depression of the vestibulary invagi- 



