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



as a feature belonging to any one species, because I have seen it in two distinct forms, 

 but I have utilised it in naming Penseus Jissurus in order to draw attention to it. 



The somites of the pleon, more particularly the first three, are each divided into two 

 portions, an anterior and a posterior, a deep groove separating them; the posterior 

 portion carries the coxal plate of the pleopod ; it is large, broad, and anteriorly overlaps 

 the posterior extremity of the carapace, and posteriorly the anterior margin of the second 

 somite of the pleon. In this it differs from the species of the Pakemonidaa, in which 

 the second somite of the pleon overlaps the one before as well as the next behind. In 

 the Penseidse the anterior three somites are never carinated, but those that are posterior 

 to them are always extremely so ; even when not produced to the form of a tooth, the 

 posterior extremity of the carinated somites is longitudinally cleft for the reception of 

 the carina of the next succeeding somite, and the telson is generally dorsally flattened or 

 grooved, and has the sides compressed and frequently fringed with small spines and hairs. 



The ophthalmopod is two-jointed, and is attached to a base that freely articulates 

 with the frontal surface or metope, which represents the first somite of the cephalon ; the 

 first joint articulates with the somite, the second with the eye. The stalk is flattened 

 in Penieus, but it is cylindrical and single-jointed in Aristeus, as it is in the other 

 families of the group. In Benthesicymiis the stalk is flattened transversely, more 

 especially on the upper side, in conformity with the plane of the surface when the 

 ophthalmopod is ensconced in the depression of the first pair of antennse ; and the 

 ophthalmus or visual extremity of the ophthalmopod is very large and reniform. In 

 some species the eyes are so arranged as to expose the surface of all the numerous 

 lenses to the light, bringing the ophthalmopoda with their blind sides contiguous to each 

 other. In Benthesicymiis and Gennadas the visual portion of the eye is not broader 

 than the stalk on which it stands ; the pigment is reduced in many species to a small 

 black or brown spot, and the lenses, which are few and not closely packed, are situated 

 at a considerable distance from the spot of dark pigment. This kind of eye appears to 

 be one of weakened power, and when at rest, or indeed at any time, has only a 

 limited range of vision, to compensate for which some species, more especially those 

 of the genus Gennadas, in which it is larger than in most others, have a supplementary 

 eye in the form of a small tubercle which eucloses a single lens. This appears to be 

 mostly adapted to those animals that inhabit the greater depths of the ocean, where 

 only the feeblest rays of light penetrate. 



Mr. John Murray ' has suggested that these secondary eyes may be, and probably 

 are, phosphorescent organs, he having seen them brilliantly luminous in some species of 

 Crustacea. 



In all Crustacea above the Entomostracous forms the first pair of antennas consists 

 of a peduncle of three joints and two terminal flagella. In some cases the outer branch 



1 Narr. Cball. Exp., p. 743. 



