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



There are five pairs of pleopoda : four consist of small and slender biramose 

 appendages, which support the ova in the female ; the fifth is strong, stiff, and sharp- 

 pointed, and helps to form the rhipidura. 



Observations. — The object of the peculiar formation of the first pair of pereiopoda is 

 very difficult to discover, except it be that of forming an operculum, so as to protect the 

 entrance, should the animal reside in a shell or enclosure of any kind ; the arms are incap- 

 able of being directed in an extended position, and therefore cannot be advanced to grasp 

 any object beyond the extremity of the antennas. The two succeeding pairs are long 

 and slender, but the last two — the penultimate and ultimate pairs of pereiopoda — possess 

 the Anomurous character of being very much shorter, and have the dactylos in the 

 penultimate almost rudimentary in size. In the ultimate pair the propodos is paved over 

 with a number of closely-packed spiculiform points of nearly equal size, which increase 

 somewhat toward the extremity — among which it is difficult to determine the dactylos. 

 These features in the pereiopoda are strongly suggestive of an approximation to the 

 Anomurous form ; but an examination of the branchial appendages reveals a character 

 that approximates to the Macrura, more especially to those belonging to the family of the 

 Thalassinidce. In Birgus, Pagurus, Cendbita, &c, to which Cheiroplatea approximates 

 most nearly in form, the respiratory organs are phyllobranchiate in character ; in this 

 genus they are trichobranchiate, the filaments being cylindrical and arranged in two 

 longitudinal rows, bearing a resemblance to those of the Astacidea, from which the)- differ 

 in the absence of the podobranchial and mastigobranchial series of the appendages, which 

 form important features in the respiratory organs of the latter. As a whole the arrange- 

 ment of the several branchial plumes corresponds more nearly with those species that are 

 generally grouped with the family Thalassinidse than with those that belong to the 

 Astacidae. 



The mastigobranchia is absent from all the pereiopoda and from both the gnathopoda, 

 and so are the podol iranchiaa. Two arthrobranchial plumes are present on each of 

 the pereiopoda and one on each of the gnathopoda : and the pleurobranchiae exist in 

 connection with the three posterior somites of the pereion, as shown in the following 

 table: — 



Pleurobranchiae, . . 1 I 1 



Arthrobranchia?, . . . . 1 1 2 2 2 2 ... 



Podobranchia.', . . . . 



Mastigobranchia', . . . . 



h. i k 1 m n o 



The plumes are generally small, and increase in size posteriorly. Of the three pleuro- 

 branchise, the last is the only plume attached to the posterior somite ; whereas the 

 penultimate and ultimate somites, besides a, pleurobranchia, carry an anterior and 

 posterior anthrobranchial plume. But the somites which support the anterior two pairs 



