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



antennal somite. The third joint articulates with the second, at points situated at the 

 internal and external angles, and externally is produced into a thin scale of large size, 

 reaching as far as the lateral angle of the carapace ; the anterior and posterior margins 

 are parallel, and the external lateral angles are anteriorly produced to a sharp point, and 

 the posterior rounded off. The fourth joint articulates with the third at points near 

 the middle of the margin of the dorsal and ventral surfaces; it is produced to a tolerably 

 sharp angle or tooth on the inner surface, but none upon the outer. The fifth or distal 

 joint articulates with the fourth by points situated at the inner inferior angle and the 

 upper exterior angle; the joint is flattened and distended to a thin plate or scale that 

 corresponds with the extent of the squamous portion of the third joint ; its anterior 

 margin is thickly fringed with hairs, and on the upper surface, near the base, just 

 beyond the articulation with the previous joint, is a slight elevation studded with numer- 

 ous small imperforate depressions which correspond with prominent points on the inside, 

 that have slightly bulbous and roughened extremities, apparently adapted for the pur- 

 pose of muscular attachment, On the external surface, in a corresponding position 

 beneath, is a circular depression, the surface of which is covered with thick, short fur, 

 consisting of hairs thickly fringed with long delicate cilia. 



The epistoma is reduced to a minimum. The cheiloglossa which articulates with it. 

 is calcareous anteriorly, and dips beneath the mandibles, which meet each other over it. 



The mandibles (PL VII. fig. 2, d) are strongly denticulate at the incisive margin, and 

 carry a rather long, slightly curved uniarticulate synaphipod; the apophysis is long and 

 rather slender, and continues beyond the articulation at the extremity into a strong- 

 calcareous process at an obtuse angle, that supplies the place of the usual muscular 

 attachment, near the molar tubercle, and enables the mandibles to open and close. 



The metastoma is a single, thick-lobed mass, that closes over and behind the mandibles 

 and first pair of siagnopoda. 



The first pair of siagnopoda (PL VII. fig. 2, e) consists of two thin curved branches, 

 having the extremity rounded and fringed with strong spines that pass into hairs at 

 the upper margin of the outer and the lower margin of the inner. The two branches 

 are sub-equal, and the margins are parallel'; at the base of the outer is a small fasciculus 

 of ciliated hairs. 



The second pair of siagnopoda (/) consists of a single truncated branch, stunted in 

 form, and excavated at the extremity, supported by a large, somewhat fan-shaped mastigo- 

 branchial plate ; the narrow portion is directed anteriorly, the whole forming an 

 efficient operculum against the exit of the water, which it has the power to confine within 

 the branchial chamber. 



The third pair of siagnopoda (g) consists of two branches and a mastigobranchial 

 plate. The outer branch is double and longitudinally angular, truncated at the apex, and 

 is connected with the mastigobranchia, being articulated with it; the inner branch 



