BRANCHIOPODA 325 



and are given free play by the reduction of the carapace so that only 

 the dorsal brood pouch remains. 



Suborder CALYPTOMERA 



Cladocera whose carapace completely covers the trunk and its limbs, 

 some or all of which are lamellate. 



The lower members of this group show affinities with the long- 

 bodied orders (Phyllopoda) of the Branchiopoda in that their trunk 

 limbs are all alike and all strain food from the water, gnathobases 

 .are present on these limbs, and the heart is comparatively long. 

 Most of the genera, however, have the trunk limbs differentiated as 

 parts of a complex apparatus in which only some of them act as 

 strainers, have lost the gnathobases of the straining limbs, and have 

 a short heart, of oval outline. 



Sida, which may be taken among weeds in pools in various parts 

 of Britain, is an example of the more primitive forms mentioned 

 above (Ctenopoda). It has six pairs of trunk limbs. 



Daphnia and Simocephalus , common British forms, found swim- 

 ming in ponds and ditches, are examples of the higher genera 

 (Anomopoda). Simocephalus (Fig. 233) differs from Daphnia in 

 possessing a cervical groove (p. 296), and in lacking a dorsal spine 

 which in Daphnia stands on the hinder edge of the carapace. The 

 following description applies to both genera. The head is bent down- 

 wards, so that the median eye and the small antennules are ventral to 

 the antennae. A large, sessile compound 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 Chirocephaliis (pp. 318, 319). The segmentation 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 dis- 

 tinct 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 represents 

 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. 234. Together 

 they form a food-gathering mechanism which is very efficient because, 

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



