3(5 THE VOYAGE OF H.M.S. CHALLENGER. 



The pleopoda are biramose, attached to a short peduncle ; the outer and larger branch 



springs from the side of the peduncle,, the other from the apex. The outer branch is 



crescent-shaped, and almost encloses the inner. It is fringed on the outer or convex 



margin with numerous long hairs of a peculiar structure. Every hair articulates at the 



base by a movable joint, and beyond a short distance from its base 



becomes multiarticulate (i.e., is broken up into numerous small sections 



or joints), and is fringed along the margin with short fine cilia. The 



inner margin of the outer plate is concave, and fringed with a few 



equidistant stiff straight hairs. The inner branch is fringed on each side 



with long pointed hairs, above which, on the side approximating the 



outer plate, is a row of equidistant straight stiff hairs ; on the inner 



side is a short stylamblys, without hairs or cilia, but furnished near 



fig. 3.— Arthrobranchia the apex with two rows of small blunt hooks, to which Sars has given 



of ocallasis ambomas, ■*- ° 



the name of cincinnuli. 



The posterior pair of pleopoda, with the telson, unite to form the rhipidura or caudal 

 fan. It consists of an extremely short base, and two broad foliaceous plates, of which 

 the outer is larger than the inner, and both are terminally fringed with hairs. 



Observations. — The jointed character of the hairs that spring from the margins of the 

 pleopoda, as well as in Ckeramus, are different from those in Callianassa both in position 

 and structure. In Callianassa the outer margin is reflected on itself so as to show a 

 smooth surface outwardly, as shown in Milne-Edwards's figure, and the hairs are all turned 

 back and pressed against the posterior margin, and are all minutely multiarticulate. In 

 Cheramus and Scallasis the margin is not reversed, and the hairs are broken into small 

 joints that are suggestive of the differentiation of character, as shown in the homologous 

 structure of Callianidea. 



Division B. 



This division contains only the genus Callianidea, of winch no species 

 was taken by the Expedition. 



Division C. 



This division contains the genus Gebia, of which no species was taken 

 during the Voyage of the Challenger. 



Family Axiid^. 



Dorsal surface of the carapace anteriorly produced to a horizontally flattened point 

 or rostrum. First pair of pereiopoda large, chelate, subsymmetrical. subequal. Second 



