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



The first pair of pereiopoda is short, not half the length of the preceding pair of 

 appendages, appears to be only four-jointed, and exhibits no sign of having any 

 prehensile power at the ultimate articulation. The second pair is longer than the first 

 and not chelate, but terminates in a feeble joint, tipped with one or two hairs. The 

 third pair is longer than the second, and, like it, not chelate, but terminates in an 

 immature dactylos. The fourth and fifth pairs are in an incipient stage of gemma- 

 tion. 



The pleopoda are in a tolerably advanced stage ; the first pair is single-branched, the 

 others biramose. The sixth pair, which helps to form the rhipidura, is well developed ; 

 the outer plate is armed near the middle of the outer margin with a sharp tooth, and 

 beyond this it is furnished with long and slender hairs. 



Length, 6 mm. (0 - 24 in.). 



Habitat— Taken on March 15, 1875 ; lat, l c N., long. 146° E.; in the Pacific Ocean, 

 north-west of the Admiralty Islands. 



This form has the branchiae undergoing development. A two-lobed plume is 

 attached to the membranous articulation between the walls of the pereion and the coxa 

 of the second pair of gnathopoda (i). 



The first (k) and second (I) pairs of pereiopoda have each a seven-lobed plume similarly 

 situated, while those attached to the third and fourth pairs are smaller, and appear to be 

 fixed to the pereion rather than to the membranous articulation of the leg. 



The stomach is visible through the carapace, and contains small masses of undigested 

 food. 



On each side of the carapace, posterior to the gastric region, there is a strong tooth, 

 which owing to compression from the mounting of the specimen is represented too high 

 in the figure ; from it a fasciculus of muscles appears to spring, and are attached to the 

 apophysis of the mandible. 



Observations. — The slide on which this specimen is preserved is labelled by 

 v. Willemoes Suhm " Sergestes cr. larva ten., W. Pacific," by which I presume that he 

 considered one specimen to be Sergestes crassus, and others to be the young of some 

 species that he proposed to name Sergestes tenuis. 



Mastigopus acetiformis. 



Carapace one-third the length of the animal. Eostrum about two-thirds the length 

 of the carapace, smooth, slender and horizontal ; a small tooth situated behind the orbital 

 angle on the dorsal surface ; none corresponding with the position of the antennas but 

 a small cusp on the fronto-lateral angle ; a small tooth stands above the mandibular 

 attachment. The anterior five somites of the pleon are subecpial in length, and all are 



