442 PROCEEDINGS OF THE ACADEMY OF [1898. 



eggs are of frequent occurrence. In such cases I was, for a long 

 time, puzzled to know how the eggs came to be fertilized. I after- 

 ward found by a study of serial sections that in the females of 

 all the species there is a seminal receptacle in the form of a con- 

 voluted tubule which opens into the oviduct, and in all mature 

 individuals this is filled with spermatozoa. These spermatozoa are 

 attached by their apices to the walls of the receptacle ; it is prob- 

 able that they receive nutriment from these walls just as they do in 

 the seminiferous tubules of the male, and that they can live indefi- 

 nitely in this position. Since there are myriads of these spermato- 

 zoa in the receptacle, and since they are carefully conserved, as is 

 shown by the facts that polyspermy rarely, if ever, occurs, and that 

 no superfluous spermatozoa are found in the egg capsules or oviduct, 

 it might well be that one sexual union would suffice for a life time. 

 In some such way as this must be explained the fact that perfectly 

 isolated females of C. fomicata lay eggs which are always fertilized, 

 though both the full grown males and females of this species are 

 perfectly sedentary. 



In the case of the other species named, the males are never im- 

 movably fixed to one spot, they are able to move about slowly upon 

 the surface of attachment, and, if detached, can obtain a new 

 foothold; their shells, also, are not distorted so as to fit irregular 

 surfaces as is the case with the females. In all cases locomotion is 

 limited to small individuals. The young of all species and of both 

 sexes crawl about freely and rapidly. In G. eonvexa individuals of 

 both sexes retain this power to a limited extent, but the large fe- 

 males of adunca, navicelloides and plana become firmly fixed, whereas 

 the males of these species remain relatively small and retain, to a 

 certain extent, their power of locomotion. The larger any individ- 

 ual becomes, the more limited are its powers of movement, and it is 

 evidently in relation to this fact that the males are so much smaller 

 than the females ; Jbecause of this marked sexual dimorphism, the 

 large and sedentary females may be repeatedly, or, in C. fomicata, 

 perhaps once for all, visited and fertilized by the smaller and motile 

 males. 



In C. plana the shell of the male is more nearly round than that 

 of the female, and is usually sharply pointed at the apex ; it is 

 thicker than an immature shell, the edges being thickened and the 

 lines of growth crowded together as is the case with the dwarfs. These 

 characters are so constant that it is usually easy to distinguish a 



