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



slender, feeble, and chelate. Like all the others except the first, there is a distinct 

 articulation between the basis and ischium. 



The third pair resembles the second in size and form, while the fourth and fifth 

 correspond in size, but differ from the preceding in not being chelate, and in having a 

 lone, simple dactylos, while the distal extremity of the propodos is furnished on the 

 inner side with a fasciculus of short hairs. 



The branchiae are similar to those of the European Neplirops norvegicus, as shown 

 in the accompanying formula : — 



Pleurobrancbiae, 



Arthrobrancbise, 



Podobranchiae, . 



Mastigobrancbiae, . . 1 



b i k 1 m n o 



In Nephrops thomsoni the podobranchiae are smaller plumes than the pleuro- 

 brancbiae, and less than their homologues in Nephrops norvegicus. 



The first pair of pleopoda (p) in the male is peculiarly modified for its recpuirements. 

 It articulates with the ventral surface of the somite, considerably within the line of the 

 posterior pans, but this is probably due to a modification of the coxal plates. It consists 

 of two joints ; the first or basisal supports the second, which, although not fused, is 

 rigidly attached to its extremity. The first is transversely triangular ; the lower angle 

 running obliquely from the base to the apex ; the second joint consists of a thin plate, 

 compressed longitudinally so as to present four carinas, the one which in the position of 

 the living animal is directed forwards, and lies against the ventral surface of the pereion, 

 having the extremity considerably produced ; it is flattened on one side to correspond with 

 the formation of the coxa of the posterior pair of pereiopoda, whde the other side is curved 

 into a deep hollow, which with the corresponding appendage forms a cylindrical tunnel. 

 At the lower anterior extremity of this semitubular plate is a small wart-like excre- 

 scence, which, under a magnifying power, is seen to be a mass of small hook-like spines 

 (cincinnuli). The upper division of the same joint is produced into a strong point. The 

 whole arrangement appears to be modified so as to be capable of assisting in the pro- 

 cess of copulation, the extended vas deferens being directed to its position by passing 

 through the channel formed by the two converging pleopoda ; the plan being similar to 

 but less perfect than that which exists in the Brachyura. 



The second pah' of pleopoda (q) consists of a basal joint supporting two foliaceous 

 plates of a long, fiat, ovate form, fringed with long, ciliated hairs ; the inner one supports 

 a short, semilunate stylamblys, fringed on the convex margin with short bans. The 

 third, fourth, and fifth pairs of pleopoda resemble the second, but possess no stylamblys. 

 The sixth pair of pleopoda forms the outer plates of the rhipidura (PI. XXV. fig. 3) ; the 

 basal joint is short, broad anteriorly and narrowing posteriorly, where it terminates in 



