XXXVI 



THE VOYAGE OF H.M.S. CHALLENGER. 



It is almost impossible to determine the home-typical relation of the several joints in 

 comparison with those of the true Crustacean leg, but it appears to me that the two inner 

 branches belong to the coxa and basis, and that the external branch is an ecphysis of the 

 second joint. 



These five pairs of appendages belong to the great dorsal shield, and are the true 

 cephalic appendages, the following pairs belonging to the pereion. 



The Second Siagnopoda. — The second pair of siagnopoda is large, variable in form, 

 situated on the outer side of the first pair, but a little behind it, and planted at the 

 anterior exit of the branchial chamber. It generally consists of 

 three or four branches, two of which are short, broad, and folia- 

 ceous, while the third is cylindrical and rod-like, and one is long, 

 broad, and membranous. 



The first joint is generally broad and short, the inner margin 

 being thickly fringed with ciliated hairs ; this joint I take to be the 

 homologue of the coxa of the theoretical leg. The second joint 

 frequently resembles the first in form and general appearance and 

 is similarly furnished with hairs, but it is usually bilobed, and the 

 marginal hairs are simple ; the third branch is short and cylindrical, 

 and tipped with one or two hairs ; on the outer side is the broad 

 and membranous plate that I take to be the homotype of the 

 mastigobranchial plate of the pereiopoda ; it is liable to vary in form 

 and size, but that belonging to Spirontocaris spinns is illustrative 

 of the most normal type (PL CVII. fig./). 

 In Homarus this pair of appendages consists of three branches or joints, of which 

 the two inner are double and foliaceous, the outer being single and tapering. In Astacus 

 the same conditions exist, but there is added on the outer side a broad and leaf-like 

 plate of semi-membranous character, and this I believe is the homotype of the 

 mastigobranchia attached to the pereionic appendages. 



In Stenopus and Spongioid this appendage is formed on the same plan, but in 

 these two genera the outer plate or mastigobranchia is produced posteriorly as well as 

 anteriorly, but is ciliated most abundantly on the anterior margin. 



Passing on to the Dendrobranchiata we find this latter condition continued, but the 

 anterior branches are shorter and broader, as in Penseus, Sergestes, and Sicyonia; the first 

 and third branches having a tendency to become rudimentary. 



In the Phyllobranchiata the same structure exists, but with a greater variation of 

 parts. The first joint generally carries the branch on the inner side, although as in 

 Athanas and Alpheus it may be seen only in a rudimentary condition ; the second joint 

 is bifid as among the Trichobranchiata, but differs somewhat in form, being deeply 



Fig. XIII.— Second 

 Siagnopod. 



