46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



formed as invaginations of the body wall at the gonopores on the 

 bases of the third pereiopods, into which open the short oviducts 

 (fig. 18 H, Spt). Inasmuch as the sperm containers are ectodermal 

 structures, insemination takes place just after a moult of the female. 

 The paired intromittent apparatus of the male crabs is a complex 

 structure composed of the penes of the last thoracic segment, and the 

 greatly modified first and second pleopods of the abdomen. Both sets 

 of organs are ordinarily concealed in a deep median depression on 

 the under surface of the thorax covered by the T-shaped abdomen 

 when the latter is turned forward in its usual position. 



In the blue crab, Callinectes sapidus, the principal member of each 

 composite intromittent organ is the first pleopod (first gonopod). 

 This appendage consists of a large flat coxopodite (fig. i8 B, Cxpd) 

 projecting laterally from the small triangular sternum of its segment, 

 and of a long tapering telopodite, or " flagellum ", with an enlarged 

 base. The thick base of the flagellum {h) turns mesally from the end 

 of the coxopodite (A, B), and the slender, tapering distal part (a) 

 extends anteriorly, overlapping its mate from the opposite side (B) 

 in the ventral groove of the thorax above the flexed abdomen. The 

 walls of the bulblike base of the flagellum are partly membranous 

 (B, D, mb), and the posterior margin of the bulb is cleft by a deep 

 groove that expands inwardly as a large internal cavity ; the enlarged 

 ends of the groove form a proximal foramen on the anterior surface 

 of the bulb (D, c) and a distal foramen on the posterior surface 

 (B, d). The groove, however, is continued from the lower lip of the 

 distal foramen along the entire length of the slender distal part of 

 the flagellum (B, gr), forming within the latter a tubular canal con- 

 tinuous with the cavity of the base and opening mesally near the tip. 

 The second gonopod (C) resembles the first in general form, but it 

 is much smaller, and has a small middle segment (Bspd) interpolated 

 between the base of the flagellum and the coxopodite ; the flagellum 

 (e) is a short curved arm no longer than the bulbular base of the 

 first flagellum. The penes of the crab arise from the mesal ends of 

 the coxopodites of the last pereiopods. Each is a long flexible tube 

 (D, Pen) traversed by the gonoduct, which opens at its extremity. 



The component elements of the genital complex of the crab are 

 intimately associated with one another, and constitute an apparatus 

 for conveying the spermatozoa from the male gonoducts into the 

 seminal receptacles of the female. In the normal condition the penis 

 of each composite organ lies mostly within the cavity of the bulbular 

 base of first flagellum, which it enters by the anterior proximal fora- 

 men of the latter (fig. i8 A, Pen) ; the end of the second flagellum 



