METAZOA CRUSTACEA. 349 



seen in a dorsal view, since they are doubled up within 

 the branchial cavity, and are attached to the movable 

 thoracic segment. 



The two arms are similar but differ greatly from 

 these organs in other genera. The sections are very 

 nearly the same in breadth and terminate in a claw, the 

 two jaws of which are about equal in size and armed 

 with sharp fine teeth. The arms extend straight out in 

 front as seen in No. 852. All these peculiarities suggest 

 that the habits of Galathea are very different from those 

 of the typical Macroura. 



An unusual modification of the external skeleton is 

 found in. Callianassa grandimana Gibbes (No. 853). It 

 is membranous and looks at first sight as if it were the 

 soft unhardened skin which appears when the crustacean 

 has just shed its crust or shell. The fossorial habit of 

 living in tunnels which it excavates in the sand is doubt- 

 less the cause for this condition of the skeleton. The 

 tunnel or "tubular domicile " extends vertically downward 

 to a considerable depth, according to Say, 1 and its com- 

 pact sides project half an inch or more above the surface, 

 like a chimney which contracts to a small opening. 



Callianassa has a long, broad abdomen which takes up 

 two thirds of the body. The cephalothorax is corre- 

 spondingly short and the" rostrum consists of two pieces 

 hinged to the carapace. It is interesting to note how 

 the terminal sections of most of the cephalothoracic ap- 

 pendages are furnished with dark brown hairs which 

 grow on these parts that perform the hardest work in 

 digging. 



The hermit crab, Eupagurus (No. 854, E. bernhardus 

 Brandt.; No. 855, the same, showing nervous system), 

 is a modified form with adaptive characters. The young 

 of some species of Eupagurus have a symmetrical abdo- 

 men like that of a shrimp, with swimming-legs on the 



1 Journ. Acad. Nat. Sci. Phila., I, 1817, p. 240. 



