312 CEUSTACEA. BRANCHIOPODA 



SUB-CLASS II. BRANCHIOPODA 



The Branchiopoda include the water-fleas (Daphnia, 

 etc.) and other forms. The body is more or less distinctly 

 segmented, and often the greater part, or sometimes the 

 whole, of it is covered by a carapace which may be shield- 

 like, as in A pus, or in the form of a bivalved shell re- 

 sembling a lamellibranch, as in Estheria (fig. 134) ; in some 

 forms there is no carapace. The number of segments in 

 the trunk (abdomen and thorax) varies very widely; in 

 some cases there may be as many as 42 ; often the division 

 of the trunk into thorax and abdomen is indistinct. On 

 the head there are generally two pairs of antennae, one of 

 mandibles, and one or two of maxillae. The trunk bears 

 several pairs of swimming-feet, which are flattened and 

 leaf-like, and their basal parts function as jaws (gnatho- 

 bases). The abdomen may be without appendages, but 

 generally appendages are present except on the posterior 

 segments. The last segment of the abdomen (the telson) 

 often bears a pair of spine-like or jointed processes forming 

 a caudal fork. Compound eyes are usually present, and 

 often also a simple unpaired eye ; the former are usually 

 sessile, but in some cases are borne on movable stalks. 



The Branchiopoda are divided into two Orders, (1) the 

 Phyllopoda, (2) the Cladocera; the latter are not definitely 

 known as fossils. 



Estheria (fig. 134). Valves equal, thin, horny ; ovate, oblong 

 or quadrilateral, united at the straight 

 dorsal border ; the apices of the valves 

 placed anteriorly, or nearly central. 

 Surface generally covered with con- 

 centric ridges or striae. Old Red Sand- „ 



. T . . . Jng. 134. Estheria minuta, 



stone to present day. Lives in fresh from the Trias x 3. 



or rarely in brackish water. Ex. E. 

 minuta, Trias, etc. 



