g4 John H. Geroui-d, 



excluded from the nephridium, while the fully grown oocytes are 

 collected there in great numbers. 



It is possible at present to give only a tentative explanation 

 of the process of separation of the larger ova from those which are 

 small and immature, and from the coelomic corpuscles. I have ob- 

 served that the nephridia in Fh. vulgare become distended with a 

 clear fluid before they take up the ova or spermatozoa, and that in 

 this fluid currents are maintained on various parts of the inner 

 surface of the nephridium, setting backward toward its free (blind) 

 end. It is probable that this liquid is sea w^ater, which has been 

 taken in through the nephridiopore. That ova in the early stages 

 of maturation probably absorb water while within the nephridium is 

 indicated by the fact of their diminished specific gravity when first 

 removed from it, or when first expelled. Such eggs [Ph. vulgare) 

 sink slowly in sea water during the course of a half hour, whereas 

 the largest ova from the body cavity fall immediately to the bottom. ^) 



I have regularly taken advantage of this fact to separate the 

 ova that are capable of fertilization from the smaller eggs and 

 coelomic corpuscles, which float in sea water and may be decanted 

 (See Appendix B). This would tend to show that the clear fluid 

 within the nephridium at such times is sea water. If eggs in the 

 earliest stages of maturation show a tendency within the nephridium 

 to absorb sea water, may it not be assumed that ova at that stage 

 are positively hydrotropic, whereas immature ova are not? On this 

 supposition we may explain why such eggs are caught up from the 

 coelomic currents into the nephrostomal region, and thence are car- 

 ried into the nephridium. 



The mature spermatozoa in the males are likewise collected into 

 the nephridia by ciliary action. The spermatozoa are inactive until 

 extruded into the sea water. 



In transparent male specimens of Ph. vulgare the nephridia are 

 often fllled with sperm, or in the female with ova, several hours 

 before egg-laying may be expected. The nephridia become enormously 

 lengthened and dilated with fluid, so that they extend backward 



1) When the contents of the coelom are stirred up in sea water, 

 the largest ova sink before the smaller ones, because, as I suppose, their 

 greater momentum enables them better to overcome the friction of the 

 water ; the specific gravity of the coelomic corpuscles , however, is less 

 than that of the eggs within the coelom. See Appendix B, 



