ISOPODA OP THE 'LIGHTNING' AND OTHER EXPEDITIONS. 103 



First maxillae furnished with a backward-directed palp, terminating in two setae. 

 Second maxillae only represented by minute, rudimentary, naked lobes. 



Maxillipeds with a falciform palp within the branchial chamber. 



First gnathopods usually well developed, with the upper visible joint large and very 

 tumid, the hand strong and chelate. In the male the whole limb, and especially the 

 hand, assumes prodigious proportions, its base so encroaching upon neighbouring 

 organs that in the adult the whole of the mouth-organs become more or less 

 absorbed. 



Second gnathopods not differing greatly in character from the peraeopods, the three 

 last joints not flattened, suited for progression. 



Gnathopods not palpigerous. 



Pleopods generally composed of a basal joint and two short setiferous lobes; rarely 

 altogether absent. 



Uropods either simple or furnished with two filaments ; in the latter case the outer 

 filament is always short, never consisting of more than three articulations, usually of 

 one or two. 



Fritz Miiller was the first to observe the remarkable change in the characters of the 

 adult male of certain members of this family, especially of the genus Leptochelia, the 

 genus to which the species called Tanais dubius by Fritz Miiller belongs. He writes : 

 — " In our Tanais, the young males up to the last change of skin preceding sexual 

 maturity resemble the females, but then they undergo an important metamorphosis. 

 Amongst other things, they lose the movable appendages of the mouth even to those 

 which serve for the maintenance of the respiratory current ; their intestine is always found 

 empty, and they appear only to live for love. But what is most remarkable is, that they 

 now appear under two different forms. Some acquire powerful, long-fingered, and very 

 mobile chelfe, and, instead of the single olfactory filament of the female, have from 

 12 to 17 of these organs, which stand two or three together on each joint of the 

 flagellum. The others retain the short thick form of the chelae of the females ; but, 

 on the other hand, their antennae are equipped with a far greater number of olfactory 

 filaments, which stand in groups of from five to seven together. ... I have 

 examined thousands of them with the simple lens, and 1 have also examined many hun- 

 dreds with the microscope, without finding any differences among the females, or any 

 intermediate forms between the two kinds of males " \ 



We are not aware that Fritz Miiller's observations which led him to believe in two 

 forms of the males in Tanais {Leptochelia) have received confirmation from any subse- 

 quent writer except Dr. Dohrn, who has described two forms of the male in Tanais 

 dubius^; but in this Prof. G. O. Sars has stated his opinion that the two males belong 



' Fritz Miiller, ' Pacts and Arguments for Darwin ' (Dallas), pp. 20-22. 

 ^ Dohrn, ' Untersuohungen iiber Ban und Entwickelung der Arthropoden.' 



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