IXBKEEDING, PARTHENOGENESIS. ASEXUAL REPKODUCTIOX 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-fertiHzation, for not infre- 

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

 the bladder of a single froof. 



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 bj- 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 Anilocra 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 indi^ddual 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 thej- generally occur 

 in great numbers together, they are probably onlj' 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 renmant, so to 

 speak, of a past stage of the modem 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 MiiUer and Yves Delage — those sac-like Ehizo- 

 eephalidffi or root-crustaceans which are altogether disfigured by 

 parasitism. The fully developed animals are hermaphrodite and live 

 partly in, partly upon crabs and hermit-crabs (Fig. 112, C, Sacc). 

 These hermaphrodites indeed fertilize themselves, but in their youth 



II. E 



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