366 ARTHROPODA 



Order I. Branchiopoda. 



The Branchiopoda are relatively large with numerous segments, leaf-like 

 appendages, long, chambered heart, and lack swimming antennae. With few 

 exceptions they are inhabitants of fresh water. According to the development 

 of the carapace they are subdivided into three families. 



i. APODID.. Body depressed, with large oval undivided carapace. Eggs 

 carried in brood capsules formed by a pair of appendages. A pus* (fig. 376), 

 Lcpidiirns* Protocaris of the Cambrian is apparently an Apodid. 2. BRAN- 

 CHIPID.E. Body without carapace, the second antennae of the male large and 

 modified for clasping the female. The female carries the summer eggs in a 

 wide 'uterus' in the abdomen. Branchipus* (fig. 382), fresh water; Artemia* 

 in brine; one has been transformed into the other by changing the water from 

 fresh to salt or the reverse. 3. ESTHERIID/E. Body laterally compressed and 

 enclosed in a bivalve shell, compound eyes fused; males very rare. Estheria* 

 Litnnadia,* fresh water. 



Order II. Cladocera. 



Like the estheriids the small Cladocera have the body enclosed in a bivalve 

 carapace, which in some is small and reaches back only over the first trunk seg- 

 ments, in others is large, enclosing the body, with a notch for the protrusion of 

 the head, while behind it terminates in a sharp spine. The head bears a pair of 

 large swimming antennae and a much smaller first pair bearing olfactory bristles 

 and, in the male, hooks for clasping the female. The body consists of few seg- 

 ments, the heart is a simple sac, and the fused faceted eyes are capable of motion 

 in a special optic capsule. The young eggs in the sexual organs always occur 

 in groups of four (fig. 383). Of these but one grows into an egg, the others 

 serving this as nourishment. Larger eggs with more yolk occur when several 

 groups fuse to form a single egg. The summer eggs arise from a single group, 

 the winter eggs from several groups of primordial ova. The space between the 

 back of the animal and the shell serves as a brood pouch. The larger winter 

 eggs one or two in number frequently remain for a while in the brood 

 chamber and are there enveloped in a peculiar shell, the ephippium, consisting 

 of two chitinous plates, like watch crystals, their edges closely appressed. 



DAPHNHXE. Shell well developed; Daphnia* (fig. 383), Bosmina* POLY- 

 PHEMID.*:. Shell small, only functioning as a brood case; head with an enor- 

 mous eye and large swimming antenna; no phyllopodous feet; marine and lacus- 

 trine. Leptodora hyalina* appears at night, sometimes in great numbers, in 

 some of our lakes. Evadne* (fig. 384), marine. 



Sub Class III. Copepoda. 



A general description of the copepods can only apply to the non- 

 parasitic forms, since many of the parasites are so degenerate (figs. 6, 388) 

 as to be recognized even as arthropods only by a knowledge of the develop- 

 ment. The sixteen somites of the body are nearly equally divided among 

 the three regions head (6), thorax (5), and abdomen (5) of the animal. 

 (In Cyclops the first thoracic segment is fused with the head, the first two 

 abdominal segments are fused fig. 7.) The last abdominal segment 

 is two-forked, forming ihefurca. While the abdomen lacks appendages, 

 the thorax bears typical biramous appendages, consisting of a two-jointed 



