ENTOMOSTRACA. 31 



The first family which we will mention is the Cyprididje, so called from its most 

 prominent genus Cypris. The family is readily distinguished from the next by having 

 two pairs of ambulatory feet and two pairs of maxillss ; the heart is absent, the eyes 

 simple (in Bairdia they are absent). The species are found in both salt and fresh 

 water, and are distributed in all parts of the globe. Cypris is a small form scarcely 

 exceeding a twentieth of an inch in length ; but, notwithstanding its minuteness, it has a 

 highly creditable ancestry, for forms usually referred to that genus are found in the ■ 

 Silurian rocks. The animal swims by means of its antennas, which are protruded from 

 the shell for that purpose, but as frequently the same organs are Used in, a mode of 

 locomotion best described by the term scrambling. The antennae are aided in the 

 latter method by the hind legs and by the abdomen, which is forked at the extremity, 

 and is used as a lever to push the auimal forward. The smaller antennae have chiefly 

 sensory functions, while a membranous portion of the maxillae by a constant motion 

 keeps up a continual current of water through the valves, thus aiding in respiration. 

 Bairdia^ another genus of this family, extends back to Silurian times. 



The family CTTHEEiDiE likewise lacks a heart ; the shell is thicker and stouter than 

 is the case with the Cyprididse, while there is but one pair of maxillae and three pairs 

 of ambulatory feet. About twenty genera, with numerous species, are known. 



The four remaining families are less important, and may be dismissed with the mere 

 mention of their names : Halocypeid^, with three genera, Cypeidinid^, with nine 

 and PoLYCOPiD^ and Cytheeellid,*, with a single genus in each. 



The young Ostracoda hatch from the egg as a modified nauplius with three pairs 

 of appendages, but possessing the bivalve shell of the adult. 



Oeder IV. — CLADOCERA. 



This group includes mostly small fresh-water Crustacea, which are enveloped in 

 a bivalve shell, and which have the antennae fitted for locomotion. Few salt-water 

 forms are known, but almost every body of fresh water Supports its myriads. Open 

 lakes, ponds, the weedy margins of ponds and streams, marshes, muddy bottoms of 

 waters, and temporary pools, all have their population la,rgely made up of members 

 of the Cladocera, whose structure is frequently modified to correspond with the habitat. 



We can best arrive' at a knowledge of the structure of this group by a study of the 

 anatomy of Baphnia, one of the most common forms in fi-esh water. 



Anatomically Baphnia is divided into a head and a body, the latter being enclosed 

 in a large bivalve shell. In the head lie the brain, liver-tubules, and eye, the latter organ 

 with its large body of pigment being the 

 most conspicuous structure. The eye is 

 freely movable by means of three pairs 

 of muscles, and is connected with the 

 optic ganglion by numerous nerves. To 

 the sides of the head are attached the 

 large two-branched antennae, from which 

 the order gets it scientific name, the an- 

 tennae being compared to the branches 

 of a tree (xMSog a branch, and xiqa?, a 



„,^ . 1-1.1 i Pig. 40. — Anatomy ol jDaphnia pulex. a. Antenna, e. 



horn). The surface which these antennae jjggs. ;». Heart, i. intestine. «, Teison. 1,2,3,4,5. 

 present to the water is greatly increased Five pairs of legs. 



