SCHIZOPODA. 467 



other hand in the females of Nyctiphanes the exopodites are 

 absent from the sixth and seventh pairs. 



The five anterior abdominal legs are biramous swimmerets 

 with a two- jointed base. An appendix internais present on the 

 endopodite in the Euphausiidae. The two anterior legs may be 

 modified as accessory copulatory organs in the males of this family. 

 In the male Siriella the endopodite bears peculiar branchial (?} 

 processes. In the females of the Mysidae (Fig. 288) all the 

 pleopods are reduced or absent (another Cumacean character). 

 The sixth form with the telson a caudal fin, as in Syncarida, 

 Stomatopoda and Decapoda. 



The nauplius eye persists throughout life in some cases 

 (Euphausia), and the stalked paired eyes are usually well deve- 

 loped, especially in the Hemitropha, which are also provided 

 with luminous organs. In several genera of this tribe and 

 also in some of the Mysidae, the compound eye is more or less, 

 completely divided into a frontal portion, with few but much 

 enlarged elements, and a lateral (Fig. 290). Moreover, as shown 

 by Chun * in the forms inhabiting great depths of the ocean (300- 

 600 fathoms), illuminated only by phosphorescent light, retinal 

 pigment is absent (cf. p. 333), while the pigment of the iris cells 

 is disposed differently in the frontal and lateral divisions of the 

 eye (pg. ir). 



The luminous organs are spherical structures having, in their 

 fully developed condition (Fig. 289 b), a transparent ' cornea ' 

 and lens in front of the lamellate body which is probably the 

 source of light, and glistening pigment behind it. They are in 

 many cases 10 in number, situated in the eye stalks (Fig. 290), the 

 bases of the second and seventh thoracic legs, and in the median 

 ventral line on the four anterior abdominal segments (Fig. 289). 

 They are supplied with muscles by which they are turned in 

 different ways, and emit a bright light either at the will of the 

 animal or as the result of direct stimulation. They are among the 

 many sources of the brilliant sparkles seen at night in the ocean. 



A closed otocyst is present in the endopodite of the caudal 

 appendages in most Mysidae (Fig. 288, Gb). 



Fig. 241 a represents the central nervous system of Euphausia, 

 which is remarkable for the distinctness of the neuromeres, but 

 in Mysis the thoracic ganglia are more closely united. 

 * Atlantis, Bibliotheca Zool. Hft. 19. 



