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



chelate than the left, which approaches the more normal form. The carpos of the 

 right side is generally formed as if it were a continuation of the propodos ; this is 

 particularly so in Callianassa, Callianidea, and Trypiea, and we presume from their 

 general resemblance to the typical genus that it is the same also in Cheramus and 

 Scoliosis, of which genera, unfortunately, our specimens have lost the first pair of 

 pereiopoda. 



The genus Gebia differs only in this respect, but in nothing else can I detect any 

 anatomical point of separation of more than generic value, and, according to Professor 

 Huxley's observations, the branchiae are of the same specific character as is found in the 

 type of this family. 



Both the podobranchia and mastigobranchia are wanting in Callianassa, but the 

 mastigobranchia is present in Callianidea in the form of slender plates fringed with hairs, 

 and at its base a small bud -like process represents the podobranchia. 



Thus it appears that in the form of the carapace, in the subchelate character of the 

 first pair of pereiopoda and the less Anomurous condition of the posterior pair, Gebia 

 approximates to Thalassina, whereas in the form of the rhipidura, and in the condition 

 of the branchise, it approaches nearer to Callianassa; it resembles Callianidea in having 

 second pair of pleopoda constructed in the same form as the third and following, although 

 fringed with articulate ciliated hairs as in Cheramus. It therefore appears correct 

 to divide this family into three divisions. 



Division A. 



The carapace is ovate, the rostrum is reduced to a small point, and the posterior 

 pair of pereiopoda is minutely chelate ; the podobranchiae, mastigobranchiae, 

 and pleurobranchiae are entirely absent ; the second pair of pleojioda is 

 slender and filamentous, and the three following are broad, foliaceous, and 

 fringed with ciliated hairs. 



Callianassa, Leach. 



The structure of the branchiae of Callianassa is so intermediate in character that it 

 may be claimed by anatomists as belonging to either the Phyllobranchiata or to the 

 Trichobranchiata, as the plumes consist of two rows of long slender filaments so closely 

 impacted together that they are flattened into plates ; but we see in Cheramus that 

 when the pressure is relieved, the filaments assume a cylindrical form, as in the 

 typical Trichobranchiata, with which the external features of the animal strongly 

 associate it. 



