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



specimen they are of considerable length, and have apparently assumed the features 

 of the permanent organs. 



The ophthalmopoda are shorter, and the scaphocerite differs in form. 



Acanthosoma dorsispinalis (PI. LXV. fig. 1). 



The specimen from which this figure is taken differs very little as to the stage of its 

 development from that represented in fig. 3, PI. LXIIL, but it differs in several 

 important points of structure. 



The carapace, exclusive of the rostrum, is about one-third the length of the body of 

 the animal. The rostrum is nearly as long as the carapace, and thickly fringed with 

 small spines. The postero-dorsal spine is long, robust, slightly curved backwards, and 

 smooth, except for the presence of one small spine on the anterior margin, halfway 

 between the base and apex; the outer antenna! spine is slender and spinous; that which 

 surmounts the mandibular area is short and denticulate, and the two lateral mareinal 

 spines are strongly serrate. 



The pleon is dorsally armed with long spines which, including that on the sixth 

 somite, are furnished with smaller spines both on the anterior and posterior margins. 

 The postero-lateral angles of the second and three following somites are also produced 

 into compound spines, of which the anterior is the largest and the posterior the smallest, 

 that of the sixth somite being reduced to a simple, narrow, sharp, spinedike tooth. 



The telson is short and curved upwards at the posterior extremity. 



The ophthalmopoda are scarcely more than half the length of the rostrum. 



The first pair of antennas is as long as the rostrum, and terminally biramose, the two 

 branches being short and single-jointed ; the basal joint, which represents the three joints 

 of the permanent peduncle, exists now as only one, while at the base the lateral enlarge- 

 ment to receive the otocyst is visible, and determined by a small, broad tooth. 



The second pair of antennas has the flagella broken off, but the terminal joint of the 

 peduncle is preserved, as well as the scaphocerite, which is long, narrow, and armed with 

 a small tooth on the outer and smooth side at a short distance from the apical extremity, 

 which, as well as the inner margin, is fringed with hairs. 



The oral appendages I have not examined, but the gnathopoda and pereiopoda are 

 fairly advanced, although as yet devoid of hairs. 



The five anterior pairs of pleopoda are undeveloped, but each somite in the ventral 

 median line is produced into a long, stout, anteriorly curved, sharp-pointed tooth; the 

 sixth somite has no such ventral tooth, and has the pleopoda fully developed, the outer 

 plate being furnished with a strong tooth near the middle of the outer margin, the rest 

 being fringed with long slender hairs. 



