GENERAL PART. 69 



has made it very difficult to discern this minute structure in the sections; 

 but there can hardly be any doubt that the small group of cells seen in the 

 mesentery in plate vi, figure 9, from a young Pentacrinoid of Tropiometra, 

 is this gonad. In the young Pentacrinoid of Compsometra serrata the primary 

 gonad is seen rather distinctly (plate xn, figure 5). In Isometra vivipara 

 it is very distinctly seen in the specimen figured in plate xxi, figure 6. In 

 Florometra it is small and indistinct. Finally, I have found it distinct also 

 in Antedon bifida. 



The present observations thus seem to show that this primary gonad 

 develops as a rule in the young Pentacrinoids of Comatulids, to be absorbed 

 very soon, the genital organs arising as a new structure in connection with the 

 axial gland. The primary gonad appears merely as a rudimentary organ, 

 an ancestral reminiscence, being of no physiological value, but of the highest 

 morphological interest. I am inclined to think that the conclusions drawn 

 by Russo essentially from this fact respecting the relations between Crinoids 

 and Holothurians are correct. 



8. THE ECTODERM ; LARVAL NERVOUS SYSTEM. 



The present observations, although checked to some degree by the unsat- 

 isfactory histological preservation of the material, are in accordance with 

 those of Seeliger, giving the curious result that the larval ectoderm dissolves, 

 the cells wandering into the mesodermal tissue. The epithelium of the disk 

 and ambulacral furrows of the adult Comatulid are derived from the epithe- 

 lium of the vestibulum. The dorsal side of the adult Antedon, and doubtless 

 of Comatulids as a whole, entirely lacks an epithelial covering, in accordance 

 with the fact that the larval ectoderm disappears. 



The remarkable character of the ectoderm in the larva of Notocrinus 

 (see page 50) is, of course, due to the special conditions of the developing 

 embryo, which must be supposed to absorb nourishment through the skin, 

 whether this nourishment consists of unfertilized, disintegrated eggs, or of 

 some other nourishing substance secreted by the mother specimen. In con- 

 nection herewith stands, doubtless, the existence in this species of special 

 glandular structures which originate as outgrowths from the ccelom and open 

 outwards in the posterior end of the larva (see page 52). It is especially 

 noticeable that in Isometra vivipara there is nothing corresponding to this. 

 The ectoderm is there of the same structure as in Antedon and Tropiometra 

 in spite of the fact that the embryo develops within a marsupium. But in 

 Isometra the embryo remains within the egg membrane till ready to swim out, 

 and thus, evidently, is unable to absorb any nourishment through the skin, 

 while in Notocrinus the egg membrane ruptures at an early stage of the develop- 

 ment, so that the ectoderm is directly in contact with the nourishing fluid. 



The larval nervous system was found to be well developed in both Tropio- 

 metra, Compsometra, and Isometra, and of the same general character as in 



