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. Ona 
transverse section numerous extremely minute cells are observed (see Pl. 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 proboscidea. 
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 proboscidea and only one male, twelve female Colossendeis 
leptorhynchus, 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 
proboscidea, and in three species of Nymphon, I observed the testis also in the 
leg of Ascorhynchus glaber (Pl. XVI. fig. 9, x), 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 afforded 
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 small 
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 
1 The genital pores of Nymphon robustwm, Bell, g, 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 Gatien 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. 
