no. 140*. PARASITIC COPEPODS—CALIGIDM— WILSON. 497 



single [Caligus alalongae, chorinemi, productus, robustus, and trachyp- 

 ieri) are very doubtful and the probability is .strongly against them. 



For instance, while Kroyer's smaller figure of C. productus shows 

 two segments, Dana's enlarged figure of the genital segment of the 

 same species shows also the free segment as single. In several species 

 {Caligus irritans, monacanthi, and veosator) the sides of this segment 

 arc indented as if for another joint, but there is no actual division and 

 the cases just mentioned are probably the same. 



Indeed, if these or any other species really had two free segments, 

 this would be sufficient ground for a generic rather than a specific 

 distinction. 



The genital segment (G. S. fig. 2) is not, as its name would imply, 

 the seat of the reproductive organs proper, but merely of the ducts 

 leading from them, in the female the internal oviduct, and in the male 

 the vas deferens and the spermatophores (see figs. 32, 33, 34). 



But since in the female the convolutions of the oviduct contained 

 within the genital segment are the place where most of the develop- 

 ment of the egg occurs, it follows that this segment is usually plump 

 and swollen. Its shape varies greatly and is indicated for each of the 

 di tie rent species in the keys on pages 555 and 615. In many of the 

 species the walls are so transparent that the structure of the internal 

 organs may be conveniently studied through them. 



The abdomen (A. fig. 2) is always narrower than the genital segment 

 (except in Caligus hirsutus), often markedly so, and is usually shorter 

 and simple. In certain species, however, it is much longer (Caligus 

 /ticc/'urus, pelamydis, scombri, and stromatei), and it is also sometimes 

 segmented, this occurring oftener in males than in females. And 

 then the abdomen in the male is relatively longer than in the 

 female, so as to give this sex a narrowed, drawn-out appearance, con- 

 trasting strongly with the plump, stocky figure of the female. The 

 external egg cases (E. S. fig. 2) are cylindrical'tubes, the substance of 

 which is secreted by a shell gland situated in the genital segment and 

 opening into the internal oviduct very near its exit from the segment. 

 The cylinder is divided into segments by cross partitions, one between 

 every two eggs, so that often when the nauplii have escaped from the 

 eggs there is left behind a sort of moulted skin which retains the exact 

 form of tlic origin,".!, but is entirely empty. When full these egg cases 

 are the most potent influence to check the ability of the female to move 

 about freely. Oftentimes they are relatively very large, and in one 

 species, Caligus diaphanus, the two strings taken together are nearly 

 :is large as the entire body. 



There are twelve pairs of appendages, namely, two pairs of antennae, 

 our pair of mandibles, two pairs of maxillae, two pairs of maxillipeds, 

 and five pairs of swimming legs, all on the ventral surface except the 



