44 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



the body. It is interesting to observe that in the Amphipoda and 

 Isopoda insemination of the female precedes or follows a moult by 

 the latter according as the spermatozoa are stored on or in a part of 

 the body not involved by the moulting process, or in a part subject to 

 removal at ecdysis. Insemination before the moult allows the develop- 

 ment in the female of special ectodermal structures, such as the 

 oostegites or brood pockets, necessary after the eggs are fertilized. 



The Decapoda vary somewhat in their mating habits, but the sper- 

 matozoa are enclosed in spermatophores, which either are attached 

 to the external integument of the female, or are inserted into an integu- 

 mental pocket {thelycum, annulus ventralis) or into a pair of ecto- 

 dermal spermathecae associated with the oviducal openings. Insemi- 

 nation of the female, therefore, takes place immediately after a moult 

 to insure against premature loss of the sperm. 



The external genitalia of male decapods include a pair of penes 

 arising from the mesal surfaces of the coxopodites of the last pereio- 

 pods, and the modified first and second pleopods, which are the active 

 intromittent organs. The genital structures retain more of the general- 

 ized form in the Macrura than in the Brachyura. In Cambarus virilis 

 the penes are small membranous lobes projecting mesally from the 

 bases of the last pereiopods (fig. 17 A, Pen), with the gonopores on 

 their posterior surfaces. The sternum of the genital segment {Stn) 

 presents a deep concavity between the coxopodites, which is continued 

 forward on the next two segments, forming a ventral channel of the 

 thorax in which are lodged the anterior ends of the gonopods. The 

 first pair of gonopods are long, rigid appendages (B, D) projecting 

 anteriorly in the thoracic channel as far as the bases of the second 

 pereiopods. The shaft of each of these appendages tapers to a slender 

 distal process (a), but from its under surface there arises mesally a 

 large lobe that splits into two proximal processes (D, b, c), the outer 

 one of which (&) has a groove on its upper surface that leads into a 

 deep cavity at the base of the lobe. The second gonopods (E) have 

 more nearly the structure of the unmodified pleopods of the follow- 

 ing segments. Each is a biramous appendage with the two arms 

 {Expd, Endpd) supported on a two-segmented basal stalk {Cxpd, 

 iTr). The larger endopodite bears a mesal lobe {d) at the end of 

 its long first segment, which, when the appendage is turned forward 

 beneath the corresponding first gonopod, clasps the latter at the base 

 of its mesal lobe. The structure of the crayfish gonopods and the 

 manner in which the appendages perform their function of sperm 

 intromission during copulation are minutely described for Cambarus 

 aMnis by Andrews (1911). The spermatozoa are received from the 



