108 



THE VOYAGE OF H.M.S. CHALLENGER. 



rudiment of a mastigobranchia. I have seen nothing of the kind in any other species, 

 and there is no ecphysis or branch attached to any other part of the appendage, a cir- 

 cumstance that is characteristic of few Crustacea, and distinguishes the species of this 

 famdy from those of other related families. 



Fig. 22. — First gnathopod of Pentacheles eu- 

 thrix ; at the base a rudimentary palpus. 

 From a drawing and no te by Willemoes-Suhm. 



Fig. 23. — Second gnathopod of Pentacheles 

 euthrix; at the base a rudimentary palpus. 

 From a drawing by Willemoes-Suhm. 



The second pair of gnathopoda (PI. XVIII. i) is likewise devoid of an ecphysis 

 attached to any of its joints, except the mastigobranchia, which is always small, and 

 sometimes rudimentary. This limb is also pediform, more decidedly so than the first ; 

 it is much longer and more slender, it terminates in a sharp unguis, and overlies the 

 oral appendages. In Willemcesia leptodactyla, Pentacheles Isevis, Pentacheles gracilis, 

 and Polycheles crucifera the mastigobranchia exists as a decided feature, capable of 

 fulfilling its duty. In Pentacheles euthrix, as shown in Suhm's fig. 23, it is scarcely 

 more than rudimentary, while in Stereomastis auriculata, Stereomastis suhmi, and Poly- 

 cheles baccata, it is reduced to an absolutely rudimentary condition. It varies a little 

 in shape, and in some it is studded with hairs. 



The pereiopoda, with the exception of the posterior pair in the male of Polycheles, are 

 all more or less chelate, and have little tendency to vary. The first pair (PL XVIII. k) 

 is very long, being generally equal to the length of the entire animal. In Poly- 

 cheles crucifera, however, it is not quite so long, a circumstance due to the shape of the 

 carpos, which is short and broad, and approximates to the more common type in Crustacea, 

 whereas in this family it is generally very long and slender — a character common to all 

 the joints to a greater or less extent, and is especially marked in the dactylos and its 

 antagonising pollex. The latter in the genus Willemcesia is armed with a strong and 

 sharp tooth, which is absent in all the other genera. 



The inner or impinging surfaces in the great chela are furnished with a sharp margin, 

 peculiar in being formed by a scries of microscopically small plates (k""), placed erect and 

 obliquely transverse. This structure, with some slight modification, appears to be common 

 to all the species. The coxa generally carries a mastigobranchia and a podobranchial 

 plume, that vary in their importance and in relative proportion. In Polycheles baccata 

 and Pentacheles euthrix the mastigobranchia is small and of great tenuity. In Steiro- 



