REPORT ON THE CRUSTACEA MACRURA. 



xxxvii 



cleft in Palamion, broad and scarcely cleft in Alpheus, Pandalus, and unbranclicd in 

 Crangon, Gnathophylum, and Nika, in which genera the third branch exists as a single 

 obtusely pointed process, and on the outer side the mastigobranchial plate projects 

 posteriorly, sometimes in a broad and leaf-like form, and sometimes as a long and narrow 

 process ; it is generally fringed with a series of long hairs that appear to have the power 

 of sweeping the branchial chamber to the most distant limits. 



The Third Siagnopoda. — The third pair of siagnopoda, or maxilliped, as we see it in 

 Homarus, is composed of four joints, of which the first has no branch ; the second 

 consists of a broad and foliaceous plate having the inner margins 

 fringed with cilia ; the third is long and narrow, with a tendency 

 to break up into joints, and beyond this there is a long two-jointed 

 branch, the distal joint being multiarticulate. In the freshwater 

 genus Astacus the structure is very similar, but the first joint is 

 produced to a short and rudimentary plate fringed with cilia on the 

 inner margin. 



In Palinurus the two inner joints are more reduced ; the third 

 is short, rudimentary and single-jointed, and the fourth consists 

 of two long slender joints, of which the second is multiarticulate ; 

 on the outer margin beyond this joint is an appendage that is 

 rudimentary in Palinurus vulgaris, two-jointed in Palinosytus 

 lalandii, and in Palinurus (?) japonicus, where it is sufficiently 

 developed to demonstrate its relationship to the mastigobranchial 

 plates of the pereiopoda. 



In the genus Hetairus (PL CIX. fig. 2g) it is developed so that 

 the true nature of the several parts can be demonstrated. The first 

 joint is broad and foliaceous, and on the posterior margin supports 

 a large plate, divided by an opaque line across the middle dividing 

 it into two parts, suggestive of one being the elementary stage of 

 a branchial plume, the other of a mastigobranchial plate. The 

 next joint supports a long filamentary branch and resembles a 

 basecphysis of the pereiopod, differing from it in having a large 

 foliaceous plate developed at its base ; beyond are two cylindrical joints forming the 

 continuation of the true limb. 



In PlesioniJca (PL CXIII. fig. lg) the morphology is still more clearly advanced, and 

 shows the double-lobed mastigobranchia divided into two distinct foliaceous plates, 

 connected at the base, just as may be seen in PL XIIb. fig. 4g ; in Phyllosoma the 

 branchial plume exists as two simple sacs, but within one the branchia is forming, while 

 the other retains the simple features of the mastigobranchia. 



Fig. XIV.— Third 

 Siagnopod. 



