420 CHUSTACEA. 



pressed Phyllopoda, whose body, with the exception of the head, 

 which projects freely, is usually enclosed in a bivalve shell. They 

 have two large antennae, which are used in swimming, and four to 

 six pairs of swimming feet. 



The Cladocera are small simply organized Phyllopods, whose 

 resemblance to the larvae of . the shelled Branchiopoda, particularly to 

 the larva of Estheria with its six pairs of legs, gives the best indica- 

 tion of the probable origin of the group. Unlike the anterior 

 antennas, which are short, the posterior are modified to form 

 biramous swimming appendages beset with numerous long setae. 

 The four to six pairs of legs are not always foliaceous swimming 

 feet, but in many cases have the form of cylindrical ambulatory 

 or prehensile appendages. The abdomen, which is ventrally flexed, 

 develops on its dorsal side several prominences, which serve to 

 close the brood pouch. It usually consists of three free segments, 

 as well as the terminal anal portion, which is beset with rows of 

 hooks. The anal portion begins with two dorsal tactile setae and 

 ends with two hooks or styles, representing the caudal fork 

 (fig. 334). 



The internal organization is simple in correspondence with the 

 small size of the body. The compound eyes fuse together in the 

 middle line to form a large, continually trembling, frontal eye, be- 

 neath which the unpaired simple eye usually remains. A special 

 sense apparatus, whose function is not quite clear, appears in the 

 region of the neck, in the form of an aggregation of ganglion 

 cells. 



The heart has the form of an oval sac, with two transverse lateral 

 venous ostia and an anterior arterial opening. Its pulsations are 

 rhythmic, and succeed one another quickly. In spite of the want of 

 arteries and veins, the circulation of the blood, which contains amoeboid 

 cells, is completed in definite tracts marked out by lacunae and spaces 

 in the body. The looped and coiled shell gland is always present. 

 The cervical gland, which functions as an organ of attachment, is less 

 widely distributed. The \ sexual glands lie in the thorax as paired 



VI, 1819 and 1820, Leydig. " Naturgeschichte der Daphniden," Tubingen, 

 1860. P. E. Miiller, "Bidrag til Cladocerernes Fortplantings historic," 

 Kjobenhavn, 1868. Gr. 0. Sars, " Om en dimorph Udvikling samt Generations 

 vexel hos Leptodora," Vidensk. Selsk. Fork., 1873. A Weismann, " Beitrage 

 zur Kenntiss der Daphnoiden," I IV.. Leipzig, 1876 and 1877. C. Claus, " Zur 

 Kenntiss der Organisation und des feineren Baues der Daphniden, Zeit. f. miss, 

 tool, Tom XXVII, 1876. C. Claus, " Zur Kenntniss des Baues und der Organi- 

 saton der Polyphemiden," Wien, 1877. C. Grobben, " Die Embryonalentwick- 

 elung von Moina rectirostris," Arleiten aus dem zool. vergl. anatom. Institut. 

 1 1 Band, Wien, 1879. 



