THE WATER FLEAS (CLADOCERA) 



677 



scope and who cannot command the resources of a university labo- 

 ratory. They are easily collected and preserved and the species 

 may be readily identified, since little or no microscopic dissection 

 is needed to make out the specific characters. Many of the Clado- 

 cera are so transparent that the internal organs can be studied in 

 detail when the animal is viewed from the side under the microscope. 

 Many of the forms can 

 readily be kept alive in 

 small aquaria, and their 

 habits observed. There 

 is still a great amount of 

 work to be done in this 

 country in finding out 

 the local geographical 

 distribution of the spe- 

 cies and the variation of 

 the variable forms. 



The suborder is di- 

 vided into two sections so 

 different that few state- 

 ments can be made of 

 them in common. The 

 first, and by far the larger 

 section, the Calypto- 

 mera, have a large, bi- 

 valve shell, which covers the body and legs. The second section, the 

 Gymnomera, includes two species in our fresh waters. These retain 

 the shell only as a brood sac; the body and legs being free. In the 

 account which follows, the Calyptomera are kept in mind. The 

 animals belonging to this group range in size from about 0.2 mm. 

 to 3.0 mm., or even more. All have a distinct head, and a body 

 covered by a fold of the skin, which extends backward and down- 

 ward from the dorsal side of the head and constitutes a bivalve 

 shell. The junction of head and body is sometimes marked by a 

 depression, the cervical sinus or notch (Figs. 1051, 1073, 1091).^ 



' The figures referred to are designed to give the specific characters rather than 

 the anatomy, which is shown only incidentally. 



Fig. 1050. Daphnia longispina. A', antennule; A", antenna; 

 AP, abdominal processes; AS, abdominal setae; B, brain with 

 optic ganglion and ocellus; BC, broodcase with developing ova; 

 E, eye, with three eye muscles of left side; E, heart with venous 

 opening on side and exit in front; HC, hepatic cecum; /, intes- 

 tine; L, legs; Af(f, mandible; 0, ovary; P^, post abdomen with 

 ana! spines and terminal claw; R, rostrum or beak; SG, shell 

 gland. (After Sars.) 



