248 



THE CRUSTACEA 



of eleven pairs of ganglia can be distinguished, all of them, however, 

 united in a common sheath of connective tissue. The paired eyes 

 are each divided into two parts in the sub-family Nematoscelinae, 

 the fronto-dorsal division having elongated ommatidia of a structure 

 supposed to be adapted for perception of very faint light. The 

 nauiDlius-eye persists in the adult, lying between the paired eyes 

 beneath the rostrum. 



No statocysts have been found in any Euphausiacea. A 

 system of very remarkable organs, formerly regarded as " accessory 

 ^<,, eyes " but now known to be lumin- 



ous organs (photospheres), is found 

 in all Euphausiacea except Benth- 

 euplimisia. As a rule, two pairs 

 of these organs are situated in the 

 coxopodites of the second and 

 seventh pairs of thoracic append- 

 ages respectively (Fig. 141, ]))■, ^n 

 unpaired series on the sternal sur- 

 face of the abdomen between the 

 bases of each of the first four pairs 

 of pleopods, and a pair, differing 

 in structure from the others, on 

 the upper surfaces of the ocular 

 peduncles. In Stylocheiron, only 

 the posterior thoracic pair, the first 

 abdominal organ, and those on the 

 ocular peduncles are present. Each 

 of the thoracic and abdominal photo- 

 spheres (Fig. 143) is globular in form, lying beneath and detached 

 from the hypodermis, and, in some cases at least, moved by special 

 muscles. The centre of the organ is occupied by a " striated 

 body " {str) composed of radiating lamellae, which seems to be the 

 actual source of light, and in which the fibrils of a special nerve 

 appear to terminate. On the inner side of the photosphere is a 

 concave reflector (r) composed of concentric lamellae, and on the 

 outer side is a lens (/), the whole surrounded, except on the outer 

 side, by a sheath of pigment {pg). The organs on the ocular 

 peduncles differ in the absence of a lens, in their incomplete separa- 

 tion from the hypodermis, and in other details of structure. 



The spermatozoa are simple round or oval nucleated cells. 

 They are transferred to the female in lageniform spermatophores, 

 Avhich are formed within the widened terminal parts of the vasa 

 deferentia. 



The ova are small in size and are sometimes carried, probably 

 only for a short time, " loose among the thoracic legs, which, with 

 their setae, form a sort of basket " (Holt and Tattersall, Euphausio). 



Fio. 143. 



Section through one of tlie thoracic 

 phosphorescent oi'gans of Nematoscelis. c, 

 cuticle ; h, hypodermis ; I, lens ; l.r, ring of 

 lamellae surrounding the lens ; n, nerve ; 

 Jiff, pigment ; r, reflector ; str, striated body. 

 (After Chun.) 



