IxVBEEEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 241 



which is also capable of self-fertilization when, as frequently occurs^ 

 it has no companion in its place of abode. But this self-fertilization 

 is always liable to be interrupted by cross-fertilization, for not infre- 

 quently there are two, three, or even four such parasites within 

 the bladder of a single frog. 



In the tape-worms, too, cross-fertilization is not excluded, for 

 there are often two or more of these animals together in the intestine 

 of a host at the same time. But even where there is only one, self- 

 fertilization on the part of the joints, that is, the sexual individuals,, 

 is prevented, and by the same device, metaphorically speaking, as in 

 the case of the oyster, for in each joint the male elements mature 

 first and the female elements afterwards. In certain parasitic Iso- 

 pods of the genus Ardlocra and related forms close inbreeding is 

 prevented in the same way — by a difference in the period at which 

 the two sets of gonads in the hermaphrodite individual become 

 mature (dichogamy). 



This is secured in a different way in Crustaceans which have 

 grown to maturity in a sedentary state, like the Cirrhipeds. These 

 animals, known as ' acorn-shells ' and ' barnacles,' are sedentary, 

 sometimes on rocks and stones, sometimes on a movable object, the 

 keel of a ship, floating pieces of wood, cork, or cane, or sometimes 

 attached to turtles or whales, and although they generally occur 

 in great numbers together, they are probably only able to fertilize 

 each other occasionally, and are therefore essentially dependent upon 

 self-fertilization. But Charles Darwin discovered long ago that 

 many of them, notwithstanding their hermaphroditism, have males 

 which are small, dwarf-like, and very mobile organisms, destined 

 only for a very brief life. These seemed quite superfluous in 

 association with hermaphrodite animals, and they have therefore 

 long been regarded as vestigial males, as the last remnant, so to 

 speak, of a past stage of the modern Cirrhipeds, in which the sexes 

 were separate. It is obvious, however, that we must now attribute 

 to them a deeper significance, for these so-called ' primordial males,' 

 although extremely transitory creatures without mouth or intestine, 

 represent a means of securing the cross-fertilization of the species. 

 What importance nature attaches to their preservation is shown 

 especially by the parasitic Cirrhipeds which have been so carefully 

 studied by Fritz Miiller and Yves Delage — those sac-like Rhizo- 

 cephalidse or root-crustaceans which are altogether disfigured by 

 parasitism. The fully developed animals are hermaphrodite and live 

 partly in, partly uj)on crabs and hermit-ci'abs (Fig. 112, C, Sacc). 

 These hermaphrodites indeed fertilize themselves, but in their youth 

 II. E 



