364 THE INVERTEBRATA 



various parts of the process of feeding, only some of them doing the 

 actual filtering off of the food particles. The gnathobases of the filter- 

 ing limbs do not project but are enlarged to bear most of the filter 

 fringe. The heart is a short sac in the first two trunk somites. 



Daphnia and Simocephalus ^ common British forms, found swim- 

 ming in ponds and ditches, are examples of this tribe. Simocephalus 

 (Fig. 244) differs from Daphnia in possessing a cervical groove (p. 332), 

 and in lacking a median dorsal spine which in Daphnia stands on the 

 hinder edge of the carapace. The following description applies to both 

 genera. The head is bent downwards, so that the median eye and the 

 small antennules are ventral to the antennae. A large, sessile com- 

 pound eye, formed by the fusion of a pair, stands in front. Above it 

 is a nuchal sense organ. Of the rami of the antennae one has four 

 joints and the other three, and both bear long, feathered setae. The 

 mouth parts are much like those of Chirocephalus (p. 356). The seg- 

 mentation of the trunk is obscure. The first two somites are fused 

 with the head, as is shown by the position of their appendages. Behind 

 these are three fairly distinct limb-bearing somites (so that there are 

 in all five pairs of trunk limbs), and then three that are limbless and 

 hardly distinguishable and a telson, which is compressed and produced 

 on each side of the anus into a toothed plate, bearing terminally a spine 

 that may represent a furcal ramus. The third free somite is longer 

 than the others and bears its limbs in the hinder part, which suggests 

 that it is the fifth of the six pairs of Sida which is missing here. The 

 limbless region is commonly known as the "abdomen". Two strong 

 dorsal processes on it close the brood chamber behind. 



The structure of the trunk limbs is shown in Fig. 245. Together 

 they form a food-gathering mechanism which is very efficient be- 

 cause, instead of all working in the same way as those of the Anostraca, 

 they are diflterentiated in adaptation to diff^erent parts of the task. The 

 third and fourth pairs form a pumping and straining apparatus (Fig. 

 246) which in principle is the same as those formed by the limbs of 

 Chirocephalus, but has for side walls the carapace, against which the 

 proepipodites play, and is closed behind by a barrier formed by the 

 fifth pair. The broad exopodites of the third and fourth pairs open 

 and close the ventral side of the apparatus as they flap to and fro 

 under the pressure of the water. The long, feathered bristles of the 

 first and of the distal part of the second pair guard the ventral opening 

 of the median gully and keep too large particles from being drawn 

 into it. The complex set of bristles upon the large endite or "gnatho- 

 base" (which corresponds both to the first and to the second endite 

 of the ideal series) in this limb play some part — exactly what is dis- 

 puted — in bringing the food to the mouth. Glands in the lab rum 

 produce a sticky secretion as in Chirocephalus. 



