412 CRUSTACEA. 



take the Cray-fish (Astacus fluviatilis) as a standard of comparison, and 

 briefly notice the principal variations from the type of structure ob- 

 servable in that species which are met with in other genera. 



(1055.) In the Cray-fish, and also in the Lobster, the secerning 

 organs or testes, when examined in situ, are found to occupy the dorsal 

 region of the thorax, lying upon the posterior part of the stomach. 



(1056.) Examined superficially, the testes would seem to form but 

 one mass, consisting of three lobes (fig. 207, act, 6); but on investi- 

 gating the minute structure of the organ, it is found to be made up of 

 very delicate secreting- tubes that give origin to two excretory ducts 

 (c c). After numerous convolutions, which form a kind of epididymus 

 (d), each duct, becoming slightly dilated, terminates by a distinct orifice 

 (/), seen upon the basal articulations of the last pair of ambulatory 

 legs. There is no intromittent apparatus visible; but, according to 

 Milne-Edwards*, the extremity of the excretory duct, by undergoing a 

 kind of tumefaction, may be protruded externally, so as to become effi- 

 cient in directing the course of the fecundating fluid. 



(1057.) In Crabs, the mass of the testis is exceedingly large, but in its 

 essential structure similar to that of the Cray-fish, and the external 

 opening of its excretory duct is found to occupy the same situation ; in 

 some genera, however, instead of being placed upon the first joint of the 

 last pair of legs, the orifices of the male organs are found upon the ab- 

 dominal surface of the last thoracic ring itself. 



(1058.) In the male Brachyura^ the so-called false feet constitute the 

 external sexual organs ; and Mr. Bate has several times taken Carcinus 

 mcenas in the act of copulation, under which circumstances he distinctly 

 saw these styliform processes inserted within the vulvae of the female. 

 These false feet consist of two pairs, the largerbeing anterior, and attached 

 to the first abdominal ring, the less, or posterior, to the second ring. In 

 all, except the edible Crab, the second pair is very small, apparently 

 rudimentary, and lie with their extremities inserted posteriorly into the 

 larger pair. But in Cancer pagurus, though slight, they are equally 

 long with the first pair, and have a joint, peculiar to this Crab, situated 

 near their centre, in addition to one, common to others, attached to the 

 basal joint. The orifice of this pair is slightly frilled : it lies posteriorly 

 against the first pair, which are the most important. These latter are 

 styliform, and attached by a hinge to a calcareous continuation of the 

 dermal membrane of the abdomen. From the first joint of the fifth pair 

 of legs a membranous tube (the vas deferens) passes out and enters at the 

 second joint of the so-called false feet, continuing through, and termi- 

 nating at the apex in an oval slit. Internally the tube is derived im- 

 mediately from the testicle. 



(1059.) Mr. C. Spence Bate believes that Crabs have more than one 



* Cyclop, of Anat. and Phys., art. " CRUSTACEA." 



f " Notes on Crustacea," by C. Spence Bate, Ann. & Mag. Nat. Hist., 2 ser. vi. p.109. 



