183 



specimen must be absorbed in some way or other. At first it is evidently 

 the other, not developing, eggs within the same ovary that serve as food 

 for the growing young, not directly, but by being dissolved gradually and 

 thus absorbed, perhaps by the digestive organs of the larva and the young 

 Ophiurid. But since these eggs have completely disappeared a long time 

 before the young Ophiurid has reached the size at which it is to be liberated, 

 it must be fed on some sort of nutritive iluid secernaled by the parent 

 specimen. It would not seem improbable that the oral tentacles of the 

 young have something with I lie absorption of this nourishing iluid to do. 

 The remarkable fact that this species, although representing one of the 

 most highly specialized cases of viviparity, passes through a distinct Plu- 

 teus-stage, more developed than many a truly pelagic larva, evidently 

 has an important bearing on the question of the phylogenetic meaning 

 of this larval form. But that is for another opportunity. 



III. Asteroidea. 



The Asteroids, upon the whole, lend themselves rather easily to artificial 

 fertilization and are among the favourite objects of experimental embry- 

 ological studies, although much less so than the Echinoids. A curious fact 

 which renders them somewhat less favourable objects for such a study is 

 this that the spermatozoa are very often found to be quite immovable 

 when taken from the testes and diluted with water, though the specimens 

 be evidently perfectly ripe and ready to empty the content of the gonads. 

 It is, of course, impossible to obtain fertilization by means of such im- 

 movable sperm. On adding, however, some drops of sodium hydroxyd 

 (NaOH) to the water and thus raising the alkalinity it is easy enough 

 to stir the spermatozoa to activity and then fertilization is easily ob- 

 tained 1 ). But it is surprising how much NaOH must sometimes be added, 

 before they begin to move. The spermatozoa of Asteroids are evidently 

 exceptionally sensible to changes in the alkalinity of the water, that is to 

 say, they cannot stand any lowering of the normal alkalinity of the water, 

 while they do not object to an amount of alkalinity considerably beyond 

 that of the water of their normal surroundings. 



The eggs are very rarely found to be fully mature when taken out of 

 the ovaries, even though the specimens look ever so ripe; they have nearly 

 always a large germinal vesicle. But if they are left for some hours in the 

 water, the germinal vesicle disappears, and they are then ready for fer- 

 tilization. 



') Comp. the author's paper "On the development of some British Kchinoderms". p. 7. 

 (Luidia citiaris). 



