184 LECTUllE XV. 



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

 on the other side of the same animal. This dimidiate hermaphro- 

 ditism *, as it is termed, has been most commonly observed in the 

 lobster, and is indicated by external characters. The generative ori- 

 fice opens on tlie last thoracic leg on the male side, in which the ab- 

 dominal plates are smaller and more simple ; whilst on the opposite side 

 they are broader and more ciliated, and the generative aperture is situ- 

 ated 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 efferent 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 the Astacus Jluviatilis the testes are blended together at the 

 middle line along the posterior half of their extent, anterior to which 

 they are separated and symmetrical : they consist of packets of mi- 

 nute contorted capillary secei'ning tubes. The vasa deferentia quit 

 the gland at the junction of its three apparent lobes : they form many 

 convolutions at the sides of the hinder part of the thoracic segment, 

 where they may be distinguished by their opake white colour. They 

 dilate into sperm receptacles in the last portion of their course, and 

 terminate at the small prominent orifices at the basal joints of the last 

 pair of thoracic legs. 



In the Mala the testis consists of an elongated and convoluted mass 

 of extremely minute vermicular tubuli, which mass is united by a 

 slender transverse commissural process with the testis of the opposite 

 side. The vas deferens is formed by the gradual enlargement of the 

 tubuli, and is disposed in a number of close convolutions, and some- 

 what suddenly dilates into a spiral seminal receptacle, which ter- 

 minates, as in the Astacus, on the basilar piece of the last pair of legs. 

 In many crabs, however, as in the Grapsiis and Ocipoda, the external 

 opening of the male organ is found on the sternal part of the last tho- 

 racic ring. The terminal part of the duct can be everted by a kind 

 of erection to form a temporary organ of intromission ; and the 

 Crustacea are singularly analogous to serpents in the double number 



* See Brande's Diet, of Science, Art. HERMArnRoi)iTE. 



