354 THE INVERTEBRATA 



The Branchiopoda are, on the whole, the most primitive class of 

 the Crustacea. This is seen in the varying and usually large number 

 of their somites, the usually small amount of differentiation in the 

 series of limbs on the trunk, the vascular system of the lower members 

 of the group (p. 348), and the nervous system of all (p. 340). Their 

 mouth parts, on the other hand, are small and simple in structure, a 

 condition in which they are not primitive but exhibit reduction. 

 Nearly all of them are, like sundry other archaic animals, of freshwater 

 habitat, and their characteristic mode of feeding is the taking, by 

 means of setae upon their trunk limbs, of particles of detritus or 

 plankton from suspension in the water. 



The primary divisions of the class have been mentioned on p. 327. 

 The most conspicuous differences between them are in the cara- 

 pace, the compound eyes, the antennae, the trunk limbs, and the 

 telson. 



The carapace is very variously developed. In the Anostraca it is 

 not present. The Notostraca have it as a broad, shallow cover over 

 the back. In the normal Cladocera ("Calyptomera") it bends down 

 at the sides to enclose the trunk as a shell, which forms a brood pouch 

 over the back. In the two groups of aberrant Cladocera which (though 

 they are probably not closely related) are together known as "Gym- 

 nomera" this shell has shrunken to a dorsal brood pouch leaving the 

 trunk partly or wholly uncovered. In the Conchostraca it forms in 

 the same way as in the Cladocera a shell, but here the head is usually 

 enclosed as well as the trunk, and there is a distinct dorsal hinge of 

 thin cuticle separating two valves which can be closed by an adductor 

 muscle situated in the maxillulary somite. Usually the carapace leaves 

 the trunk free within it, but in the Cladocera it fuses with two — in 

 Leptodora (p. 368) with all — of the thoracic somites. 



The antennae, which in the Nauplius are biramous and natatory, 

 retain this condition in the adult of those forms (Diplostraca) in 

 which the enclosing carapace has deprived the trunk limbs of the 

 swimming function, and also in the extinct Lepidocaris (Lipostraca). 

 In the recent Anostraca the antennae are stout but uniramous and not 

 natatory; in the male they are adapted to clasping the female. In the 

 Notostraca, which apply the head to the ground in feeding, they are 

 reduced to uniramous vestiges. 



The trunk limbs (except in the aberrant Cladocera which constitute 

 the Gymnomera) are phyllopodia (p. 336) which bear on the median 

 side endites furnished with feathered bristles and on the outer side, 

 besides the exopodite or flabellum, a thin-walled branchia and often 

 also one or two proepipodites. With these appendages the Anostraca 

 and Notostraca swim, and all members of the class breathe and gather 

 food. Beating rhythmically forward and backward with a movement 



