28 LECTURE XV. 



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

 which they are separated and symmetrical : they consist of packets of 

 minute contorted capillary secerning 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 tliey 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 orifice at the 

 basal joints of the last pair of thoracic legs. 



In the Maia 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 termi- 

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

 In many crabs, however, as in the Grapsus and Ocypoda, the 

 external opening of the male organ is found on the sternal part of 

 the last thoracic ring. The testes in the common crab {fig- 132, I) 

 consist of a net-work of very small seminiferous tubes, occupying the 

 lateral portions of the cephalothorax, which gradually increase in 

 size until they form the long sperm-ducts: these (ib. d) form 

 numerous convolutions on each side of the stomach, and dilate into 

 large ejaculatory ducts before opening upon the plastron. The ter- 

 minal 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 as well as the structure 

 of this part. Certain appendages of the first and second abdominal 

 rings in the male crabs are probably connected as exciting organs 

 with the sexual function. 



The spermatic fluid is white and sometimes opalescent : the sperm- 

 cells present varied and remarkable forms. In the Cyclops they are 

 oval corpuscles ; in Cypris they are large, filiform, and flexuous. In 

 the Isopods and Amphipods they are long filaments pointed at both 

 ends, or with a slight thickening at one end : they are motionless. 

 134 In the Decapoda the nucleus 



of the sperm-cell developes 

 one or more filiform pro- 

 cesses, which project from 

 the cell itself. In fig. 134, 



Spermatozoa of Crustacea, higl.Iy magnified. ^ showS the form of the Sper- 



matozoa in the Grapsus marmoralus ; u and c, that in Pagurus 

 oculatus ; and d, that in IHsa tetraodon. 



