382 COMPARATIVE ANATOMY CHAP. 



chance of fertilisation, which the attached or parasitic mode of life offers to sexually 

 separate animals. To explain the occurrence of a hermaphrodite condition in 

 general, we must assume (an assumption not without foundation) that the rudiments 

 of the germ glands are indifferent, that in one case, under certain unknown circum- 

 stances, they may develop as testes, in another as ovaries, and in others again pro- 

 duce ovaries as well as testes. 



In the Cirripedia, the attached and parasitic modes of life are evidently extremely 

 old. If the view that they are descended from Copepoda-HkQ forms is correct, then 

 the ancestors of the Cirripedia, when first the attached or parasitic modes of life 

 appeared, were probably sexually separate, and dimorphic with small, free-moving 

 males, as indeed is still the case in many parasitic Copepoda. The attached and 

 parasitic modes of life then became more and more pronounced in the female, and 

 caused the appearance of hermaphroditism. The males, in the meantime, who had 

 also taken on the Cirripede character, remained as dwarf males, and so the possibility 

 of occasional cross-fertilisation was preserved. In most Cirripedia the males have 

 probably in time disappeared, and the purely hermaphrodite condition has become 

 fixed. In others, the dwarf males proving under certain conditions sufficient for 

 ensuring fertilisation, have apparently led to the disappearance of hermaphroditism 

 and the reappearance of sexual dimorphism. 



In the Isopoda the sexual relations have probably developed in quite another way. 

 How protandrous hermaphroditism arose in the Cymothoidea is indeed, at present, 

 uncertain. But we can perhaps imagine the rise of the sexual relations in the Crypt- 

 oniscidce and Entoniscidce in this way ; these animals were originally, like the 

 Cymothoidea, protandrously hermaphrodite, then in time some of the larvae developed 

 only to the male stage and became larval or degenerate males. 



In the gill-inhabiting Bopyridce the male stage, in the originally hermaphrodite 

 individuals, must have been suppressed, as the dwarf males sufficed. 



In the Amphipodan species Orchcstia, the curious fact has been established that 

 a certain part of the germ layer of the testes of the male produces eggs, while the 

 other parts produce spermatozoa. The eggs, however, never, or only in exceptional 

 cases, reach the exterior, and in any case do not develop further. The above fact, 

 which does not stand alone, is at present unexplained. 



XIII. Parthenogenesis Cyclic Reproduction. 



Parthenogenesis occurs in the Crustacea only in the Phyllopoda, viz., in Esthcria 

 and Apus (see note, p. 381) among the Branchiopoda and in the Cladoccra. The males 

 are much rarer than the females, and in the Cladocera appear only periodically in 

 autumn. 



The thin-shelled summer eggs develop parthenogenetically, and in many Phyllo- 

 poda in summer there is a succession of generations of females multiplying partheno- 

 genetically. The larger hard-shelled winter eggs, on the contrary, which are supplied 

 with more nutritive yolk and are laid in autumn, require fertilisation. 



XIV. Ontogeny. 



We can here present only a selection of the observations on the ontogeny of the 

 Crustacea, which are so numerous and have such an important bearing on general 

 morphological and biological questions. We shall first briefly describe the develop- 

 ment of the outer body form of some few Crustaceans which go through a long 

 process of metamorphosis, and then give a sketch of the development of their inner 

 organisation. 



