130 THE VOYAGE OF H.M.S. CHALLENGER. 



Of course the non-existence of a hardly distinguishable structure cannot be con- 

 sidered proved merely because the structure has not been observed, even in numerous 

 specimens of the same species. But, on the other hand, if the structure is very 

 easily seen in some specimens, we are at liberty to conclude that it is absent in those 

 cases where it was not observed. Moreover, it is hardly possible that in those 

 cases where the pores are only observed on the two hindmost legs, they should be 

 present also on the foremost, and from some unknown circumstance should continually 

 escape observation.' 



In regard to the microscopical structure of the testis I have not much to say. On a 

 transverse section numerous extremely minute cells are observed (see PI. XXI. fig. 12), 

 while that part of the testis which adjoins the longitudinal canal has lost its cellular 

 structure, and shows a rather granular condition. Whether these granules are the sper- 

 matozoa, or whether the numerous globular bodies each furnished with a filamentary 

 appendage on one side which I once observed, when pulling to pieces with needles a part 

 of the testis, are the spermatozoa, cannot, of course, be ascertained from animals in 

 alcohol, even when preserved so well as the Challenger specimens are. Only in mature 

 animals do the male organs attain the development figured for Colossendeis prohoscidea. 

 This, most probably, is only the case during a short period of the year, considering 

 moreover that the males in most species seem to be less numerous than the females 

 (ten female Colossendeis 2^'>'ohoscidea and only one male, twelve female Colossendeis 

 lei^torhynchus, and only one male, &c.), it cannot be wondered at that the number of 

 species in which I could investigate these organs was limited. Except in Colossendeis 

 prohoscidea, and in three species of Nymp)hon, I observed the testis also in the 

 leg of Ascorhynchtis glaher (PI. XVI. fig. 9, n), but here only in the fourth joint of 

 the leg. 



In the other specimens which I consider as males, the only means I had to make 

 out the sex consisted in looking for external sexual characteristics, such as are afi"orded 

 by the dimensions of the genital pores and the condition of the thighs. Large genital 

 pores and swollen thighs are characteristic of the females ; slender thighs and smaU 

 pores, very often not present in the first or in the first two pairs of legs, are characteristic 

 of the males. Moreover, a transverse section of the thigh of one of the legs is easily 

 made, and does no injury worth mentioning to the specimen. When in such a section 

 no ovary is observed, so far as my experience goes, it is almost certain that the 



I The genital pores of Nymphon robustum, Bell, (J, are tolerably large, and are easily observed with the aid of a 

 magnifying glass. Yet I have examined large specimens of this species (dredged in the Barents Sea), where these pores 

 were not observed, even when investigating the joints with the microscope. As I was convinced of the exactness of 

 this observation, I felt greatly puzzled with it at first ; afterwards on reading a paper of Schobl in the Archiv f. Mikro- 

 skop. Anatomie, Bd. xvii., 1880 (Ueber die Fortpflanzung isopoder Crustaceen) I found that this author admits that in 

 the females of these Crustaceans, the genital pores are only present at a certain period, and are totally wanting during 

 the rest of the year. Perhaps there are male Pycnogonids which have the same peculiarity. 



