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



In some genera, such as Thaumastocheles (PL VII. fig. Ig), Willemoesia 



(PL XVIII. fig. g), Pentacheles (Fig. 21, p. 107), and Polycheles (Fig. 32, p. 125), what I 

 take to be the basecphysis is developed at the extremity in the form of a freely movable 

 leaf-like plate, which probably quivers under the action of the expiratory current from 

 the brancbial chamber. 



In Pasiphasa a similar condition also exists, but in a more rudimentary state, as shown 

 on PL CXLI. fig. Ig. 



The First Gnathopoda. — The first pair of gnathopoda assumes a more leg-like 

 character than either of the preceding appendages, but it is not entirely pediform until 

 we come to the aberrant Schizopoda, and the still more distant Amphipoda. 



It is generally formed of five joints, but in some genera there are six, and in a few, as 

 in Nephrops thomsoni (PL XXVI. h), there are seven joints. In general character it is 

 usually short, wide and thin, the three distal joints being reflexed on the inner side, 

 and the dactylos is generally broad and fiat. The larger the number of joints the 

 narrower they are, and the more pediform is the whole appendage ; but when the 

 joints lessen in number, the reduction is made by the coalescence of the meros and 

 ischium, and sometimes the basis also, into one, and by the absorption or loss of the 

 dactylos. Attached to the basis is an ecphysis that is generally long and two-jointed, 

 the distal joint being multiarticulate ; sometimes it is only single-jointed, as in 

 Oplophorus (PL CXX VII. fig. \h) and in Nephrops, where it puts on a somewhat rudi- 

 mentary appearance. I do not remember an instance in which it is altogether absent, 

 unless it be so in Pasipheea (PL CXLI. fig. h). The coxa almost universally has a 

 mastigobranchial plate attached, which is generally of small size and varies in form, 

 and has very constantly a branchial plume attached to it, and occasionally a second or 

 arthrobranchial plume attached to the membranous articulation. 



In the Astacidse branchial filaments are attached to the outer surface of a large 

 membranous plate that appears to resemble the mastigobranchia. 



The Second Gnathopoda.— The second pair of gnathopoda is more perfectly pedi- 

 form than the first, but varies to a greater extent in the number of its joints. In 

 the Trichobranchiata it consists of seven joints, in, I believe, every genus, not 

 excluding the Stenopidse. This circumstance is the more remarkable in the Palinuridas 

 and the Scyllaridse, as in these two families all the pereiopoda have only six joints 

 each. 



In the Dendrobranchiata there are also seven joints ; and as the leg becomes longer 

 and more slender in its gradual passage from Penseus, through Haliporus, Sergestes, and 

 Lucifer, it assumes more closely the character of the succeeding simple legs. 



In the Phyllobranchiata the number of joints is generally limited to five, this diminu- 



