Chap. HI. GENUS IDYIA. 285 



crowded upon the surface of the spermatic sacs, so that all the ambulacral rows 

 appear oue-sided, on account of the prominence imparted to them by the pigment 

 cells crowded over the spermatic sacs. Moreover, the ovarian and the spermatic 

 sacs are developed on opposite sides in adjoining ambulacra, so that proximate sides 

 of different ambulacra have the same kind of sexual organs, while alternate inter- 

 ambulacra have different kinds, the total arrangement being such that ovaries occupy 

 the anterior and the posterior interambulacra, as well as the lateral interambvdacra 

 in which trend the coeliac tuljes, while the four intervening interambulacra are occu- 

 pied by sjjermaries. The sexual sacs begin to appear early in August or in the 

 latter part of July. They are filled with eggs and spermatic cells in the latter 

 part of August; and at that time, in the larger specimens, these may be seen 

 circulating in the ramified tidies arising from the amljulacral tubes, which soon fill 

 so completely with eggs (PI. II. Fig. 6) as to appear like blood discs in a blood- 

 vessel. Owing to the ramifications of the ambulacral tulles and the extension of 

 the ovisac in the shape of similar branches extending into the spherosome, wliile 

 the spemiatic sacs communicate only with the main tubes of the ambulacra, it 

 follows that the contents of the spermaries are emptied into the ambulacral tubes, 

 and through them circulated into the ovarian sacs as soon as the eggs begin to pass 

 into the ramifications of their pouches, and, finally, eggs and spermatic particles are 

 lodged together in the ramifications of the chymiferous system, wliich penetrate the 

 spherosome, where the eggs remain enclosed until the sjiherosome itself is broken 

 up and decomposes, when the eggs and the young, in various stages of development, 

 are set free. This constitutes a marked difterence from Pleurobrachia and Bolina, 

 in which the eggs are only moved to and fro through the main chymiferous tubes. 

 The central eye-speck (PI. II. Figs. 3, 8, 9, and 18) has the same structure 

 as in Pleurobrachia and Bolina, and may be so easily observed, that, were there 

 distinct nerves connected with its bulb, I could hardly have failed to see them. 

 That the eight narrow branches converging under the base of its bulb (PI. IT. Fig. 

 3) are not nerves, but a direct prolongation of the rows of locomotive fringes, 

 presenting in their abactinal extension (PI. II. Figs. 8 and 0) the same character 

 as on their actinal prolongation {Fig. 17), is easily ascertained ; and the circumstance, 

 that while they are plainly visible at the two extremities of the rows of loco- 

 motive flappers nothing of the kind can be seen under them, not even when 

 the ambulacra are examined from their inner surface as in Fig. 10, shows distinctly 

 that they form a part of the system of locomotive flappers. But why they should 

 reach the base of the eye and terminate there is not so easily imderstood; unless it 

 is to establish a connection of some kind between sight and locomotion, in the same 

 way as the eye-specks of the Echinoderms are placed in the prolongation of the 

 ambulacra. This connection seems to me an additional evidence that the eye-speck 



