52 REPORT — 1855. 



In JEgina they are long and slender, and furnished on the outer side of the 

 neck with a small articulated scale, the rudiment of the undeveloped leg 

 (fig. 6). 



Organs of Generation (male). — The dissection of these organs requires 

 much care ; the most distinct that we have been enabled to make out were in 

 a specimen oi' Sulcator arenarius, sent us by our most valued correspondent, 

 the Rev. G. Gordon, taken in Moray Frith. This specimen was so exqui- 

 sitely transparent, that we could readily detect the white patch of the testes 

 with unassisted vision ; and by cautious dissection under the microscope, 

 we were enabled to trace the connexion between them and the external 

 organs*. 



The testicles are large, opake, oblong organs, being in breadth about 

 equal to half their length ; they are situated on the dorsal aspect, immediately 

 beneath the dermal tissues, occupying a position under the sixth and seventh 

 segments of the pereion (thorax) (PI. XXI. fig. 1). 



From the posterior extremity of each, deflecting one to the right, the other 

 to the left, a vas deferens proceeds towards and enters into the first joint of 

 the seventh pair of legs (figs. 2 and 3), and again passes out and terminates 

 in an external penis ; but whether intromittent or not we have hitherto failed 

 to discover, though we believe it is not. We have had Gammartis gracilis 

 long in keeping, and watched them in their habits much ; but have never 

 detected any communication between the sexes which could admit of a 

 direct passage of the penis into the vulva, which latter organ we have not 

 yet discovered in the normal Amphipoda. 



The male appears to grasp the opposite sex by one of its strong subche- 

 liform gnathopoda, by the insertion of the claws beneath the anterior edge 

 of the first segment of the pereion (thorax), whilst another is inserted be- 

 neath the posterior margin of the fourth or fifth. Thus grasping the female 

 by the i^ack, it draws it into immediate contact witli the ventral surface of 

 itself. In this attitude, more or less firmly compressed, they swim and rest 

 alternately for days, or perhaps, as we believe, a very much longer period, 

 without any apparent closer communication. 



H' tlie two be driven asunder by any fear of danger, as has been performed 

 by us for the value of the observation, the female seeks a place of shelter, 

 while the male swims more actively about; and we have noticed, that should 

 it after a few moments swim within a little distance of its late mate, it 

 instantly becomes aware of the circumstance, and having passed the spot, 

 will turn abruptly back, seek her out, and seize her with avidity from amidst 

 several others, and immediately after securing, strike her with two or three 

 strong lashes of the tail. The female rolling herself up in fear is so carried 

 oflT by her more powerful mate. 



This contact between the sexes is either occasionally repeated or may last 

 through the wiiole period of incubation, as we have frequently taken them 

 coupled in this manner, even when the matured young have been sufficiently 

 advanced as to leave tlie care of the parent. We are induced from this fact 

 to believe, that a series of broods are producible ijom the same parents during 

 the year, and that the erotic state of the female may exist during the incu- 

 bation of any previous brood. 



The penis is a soft Tnenibranous tube, the external continuation of the 

 vas deferens, with the probable capability of erection (PI. XXI. figs. 1, 2, 3 a). 

 The oritiee occupies but scarcely half of the diameter of the extremity of the 

 tube, and most probably has the power of closing itself voluntarily. This 

 remark is true both in Gammarus and Sulcator, in which latter the organ is 

 * The observations of De Siebold on this organ chiefly relate to the Isopoda. 



