u CLADOCERA 27 
trum with a movable spine; thoracic limbs with accessory 
lappet on the exopodite. L. seliqua, G. O, Sars—Cape Town. 
Cyclestheria,! G. O. Sars. CO. hislopt, Baird—Queensland, India, 
East Africa, Brazil. 
Sub-Order 2. Cladocera. 
The Cladocera are short-bodied Branchiopods, 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; 
Leptodora hyalina, the largest, is only about 15 mm. long, while 
many Lynceidae are not more than 0:1 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 (¢f. Fig. 10). 
The compound lateral eyes fuse at an early period of 
embryonie 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. 15). 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 Simocephalus (Figs. 9, 10), Daphnia, ete. 
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 
1 Sars, Christiania Vidensk. Forhand. 1887. For Australian Phyllopods, see 
Sars, Arch. f. Math. og Naturvid. xvii., 1895, No. 7, and Sayce, Joc. cit. p. 36. 
