CRUSTACEA 317 



Class BRANCHIOPODA 



Free Crustacea with compound eyes ; usually a carapace ; the mandi- 

 bular palp very rarely present and then as a minute vestige; and at 

 least four, usually more, pairs of trunk limbs, which are in most cases 

 broad, lobed, and fringed on the inner edge with bristles. 



The carapace is in this class very variously developed, presenting 

 a characteristic condition in each of the principal sections of the group. 

 In the Anostraca it is not present. The Notostraca have it as a broad 

 shield 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 aberrant Cladocera Gymnomera 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. 

 Usually the carapace leaves the trunk free within it, but in the Clado- 

 cera it fuses with two (in some Gymnomera with more) of the anterior 

 thoracic somites. 



Except for one small and aberrant branch (the Gymnomera), all 

 the Branchiopoda have as trunk limbs phyllopodia (p. 300) which 

 bear on the median side endites fringed with long 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 some 

 of them (but not the Cladocera or Conchostraca, which have natatory 

 antennae) swim, and all breathe and gather food. Beating rhythmi- 

 cally forward and backward with a movement which each pair starts 

 a little earlier than the pair in front of it, they cause, by a pumping 

 action which shall be described presently, a flow of water into the 

 median gully between them, outwards into the spaces between each 

 limb and its neighbours in front and behind, and then backwards, 

 bringing particles of food, bathing the branchiae, and causing, in the 

 Anostraca and Notostraca, forward movement of the body. Feeding 

 is effected by the apparatus on the inner edge, which varies in detail 

 with the food. Some have low endites with long and close set fringes 

 which strain off particles from the stream just mentioned. The 

 Anostraca and Cladocera Calyptomera best illustrate this arrangement, 

 which varies in complexity and in the higher Cladocera, such as 

 Daphnia (see below), becomes very elaborate and confined to a 

 correspondingly small number of limbs. Those Branchiopoda in 

 which the endites are more prominent and the fringe less continuous 

 have denser food material to collect — either ooze, as in many 

 Conchostraca, or solid objects as in the Notostraca, which have 



