CRUSTACEA. ' 327 



the first pair of thoracic limbs, which are short, swollen, and hooked 

 in the males, while in the females they are long, straight, and 

 pointed. On the first of the abdominal respiratory limbs there 

 is a prominent tubular appendage, on which the sperm-duct ter- 

 minates ; in the female there is a simple perforation at the corres- 

 ponding part. 



No instance of parthenogenesis has been determined in the higher 

 sub-class of Crustacea, called Malacostraca ; but, like other Arti- 

 culata subject to periodical ecdysis, they have the power of re- 

 producing lost extremities, as has been already explained. There 

 is no propagation by spontaneous fission or gemmation : every 

 species is developed from ova formed by organs peculiar to one 

 series of individuals, and impregnated by the fertilising product 

 of organs as peculiar to another series. But, although the male 

 and female organs are never naturally combined in the same indi- 

 vidual, accidental or monstrous hermaphrodites occasionally occur, 

 in which the male organ is developed on one side, and the female 

 organ on the other side of the same animal. This dimidiate her- 

 maphroditism has been most commonly observed in the lobster, 

 and is indicated by external characters. The generative orifices 

 open on the last thoracic leg on the male side, in which the ab- 

 dominal plates are smaller and more simple; whilst on the oppo- 

 site side they are broader and more ciliated, and the generative 

 aperture is situated on the middle or third ambulatory leg. This 

 singular kind of malformation seems to depend upon the very 

 slight connection, or want of connection, between the right and left 

 generative organs of the same individual, whether male or female. 

 The external apertures are always distinct on each side ; and when 

 a combination of the right and left generative organs does occur, it 

 is by a partial union of the two testes, or of the two ovaria. 



In the male Cymothoa both the essential and efierent portions of 

 the male apparatus are distinct on each side : the testis is here much 

 simplified, and consists of three elongated pyriform vesicles forming 

 a common tube by the union of the short vasa deferentia, which 

 arise respectively from the great end of each vesicle. In this and 

 some other of the Isopoda, the disproportion between the male and 

 female is still retained ; and is almost as extreme in the parasitic 

 Bopyrus as in the Lernaeans. 



In Squilla, the testes are ramified glandular lobes, from which 

 the sperm-ducts pass off laterally to terminate each in a hollow 

 evertible penis at the base of the last thoracic leg. 



In Astacus fluviatilis the testes aie blended together at the 



Y 4 



