CLADOCERA 37 



trum with a movable spine ; thoracic limbs with accessory 

 lappet on the exopodite. L. siliqua, G. O. Sars — Cape Town. 

 Cyclestheria,^ G. O. Sars. G. hislopi, Baird — Queensland, India, 

 East Africa, Brazil. 



Sub-Order 2. Cladocera. 



The Cladocera are short-bodied Branohiopods, with not more 

 than six pairs of thoracic limbs. The second antennae are 

 important organs of locomotion, and are nearly always biramous ; 

 the first antennae are small, at least in the female ; the second 

 maxillae are absent in the adult. The carapace may extend 

 backwards so as to enclose the whole post-cephalic portion of the 

 body, or may be reduced to a small dorsal brood-pouch, leaving 

 the body uncovered. 



The Cladocera or "Water -fleas" are never of great size; 

 Zeptodora liyalina, the largest, is only about 15 mm. long, while 

 many Lynceidae are not more than O'l or 0'2 mm. in length. 



The head is bent downwards in all the Cladocera, so that 

 parts which are morphologically anterior, such as the median 

 eye and the first antennae, lie ventral to or even behind the com- 

 pound eyes and the second antennae (cf. Fig. 10). 



The compound lateral eyes fuse at an early period of 

 embryonic life, so that they form a single median mass in the 

 adult, over which a fold of ectoderm grows, to make a chamber 

 over the eye, like that found in the Limnadiidae, except that it is 

 completely closed. The fused eyes are generally large and con- 

 spicuous ; in some deep-water forms the retinular elements of 

 the dorsal portion are larger than those of the ventral (e.g. 

 Bythotrephes, Fig. 13). In one or two species which live at 

 very great depths, or in caves, the eyes are altogether absent. 



The appendages of the head are fairly uniform, the most 

 variable being the first antennae. In the females of many 

 genera the first antennae are short and immovable, consisting of 

 a single joint, with a terminal bunch of sensory hairs, and often 

 a long lateral hair, as in Simocephcdus (Figs. 9, 10), Daphnia, etc. 

 In the female Moina (Fig. 16) they are movable, as they are 

 in Ceriodaphnia and some others; in Bosmina (Fig. 22) and 

 many Lyncodaphniidae they are elongated and imperfectly divided 



^ Sars, Ohristiania Vidensk. Forhand. 1887. For Australian Phyllopods, see 

 Sars, Arch. f. Math, off Naturvid. xvii., 1895, No. 7, and Sayce, loc. cit. p. 36. 



