Sludie'; on riuiriin' Osti'acods 4()."i 



seem ciiriou.s, as regards the two strongly lengthened bristles in the male, that tiiey should b(! 

 homologized with the ventral of the two longest bristles in the female and the comparatively short 

 bristle, which points forward almost at a right angle, of this sex. At the first glance it may 

 perhaps seem more reasonable to assume that the two longest bristles in the male correspond 

 to the two longest bristles in the female. The following three reasons may be given in support 

 of the former explanation. In the first place the relative position of the bristles. As is shown 

 by a comparison between the strongly magnified part of the male and female first antenna 

 reproduced for A. norvegica (figs. 7 and 8), the two strongly lengthened bristles of the male 

 (denoted by c and f) have quite the same position as the posterior of the two longest bristles 

 and the bristle that is pointed forward at about right angles on the same limb of the female; 

 the former is situated posteriorly on the joint, the other laterally somewhat behind the strong 

 end claw. An argument that is perhaps still stronger is to be obtained from embryology. 

 During the last larval stage the male and the female first antennae are of almost precisely the 

 same type; dimorphism can, however, be observed in three bristles. Two of these, the one 

 situated farthest back on the joint and the one situated laterally on the joint somewhat behind 

 the strong end claw have, it is true, the same orientation as the correspondingly situated 

 bristles on the first antenna of the mature female, but are somewhat longer comparatively 

 and are, in addition, characterized by a very marked increase in the number of the sensorial 

 filaments; cf. fig. 12 of A. Grimaldi. The remaining one of these three bristles, the one that has 

 the same position as the anterior one of the two longest bristles of the mature female, has scarcely 

 increased in length and is distinguished by a very slight increase in the number of sensorial 

 filaments. While the latter bristle in the mature female is of quite or practically quite the same 

 type as the posterior bristle on this joint, it is thus in the male, even at this stage, of a type 

 differing exceedingly from it. On the other hand the posterior bristle on the joint and the 

 bristle that points forwards at right angles are both modified in the same direction in this male 

 larva. (Such a modification of these bristles can, as a matter of fact, be already traced in male 

 larvae in stage II, but not earlier. The female larvae, on the other hand, do not show any 

 modification of this sort at all; they are, on the contrary, very close to the type of the mature 

 female.) Finally a third reason: As is shown by the description of the genus given above, the 

 bristle that points forward at right angles on the first antenna of the female is characterized by 

 the fact that its sensorial filaments issue from the posterior (-ventral) side of the bristle, while 

 all the other distal bristles on this antenna are distinguished by the fact that their sensorial 

 filaments issue along the anterior side. In the mature male too only one of the distal bristles 

 has sensorial filaments along the posterior side; this bristle is the anterior of the two stronglv 

 lengthened bristles. This is naturally a rather strong argument in favour of the homologi- 

 zation of the bristle that points rectangularly forward in the female with the anterior of the 

 two strongly lengthened bristles in the male. The homologization of the four (five) other distal 

 bristles scarcely seems to need any additional reasons. The situation of these bristles is quite 

 identical in the two sexes and even the difference in type is onlv very slight. 



As appears from the genus description given above the exopodite of the second antenna SeciKt (tui.-niKi. 

 is distinguished in the forms of this genus examined by me by marked dimorphism. This organ 



