322 THE INVERTEBRATA 



concentration of salt, and some of its minor features change with 

 the degree of the concentration, so that it has been described under 

 different specific names. It differs from Chirocephaliis in having only- 

 six abdominal somites and in the form of the antennae of the male. 



Lepidocaiis (order Lipostraca), a minute, blind, freshwater form 

 from the Middle Devonian, was closely related to the Anostraca, but 

 differed from them in the following, among other respects. It had 

 biramous antennae which recall those of the Cladocera ; a clasping 

 organ on the maxillule of the male, instead of on the antenna; and the 

 trunk limbs without branchiae and differentiated into two sets — the 

 first three pairs adapted for gathering food, with gnathobase and with 

 the last endite directed inwards and the exopodite lateral, and the re- 

 maining pairs adapted for swimming, with the last endite and the 

 exopodite directed distally side by side at the end of the limb. 



Order NOTOSTRACA 



Branchiopoda with a carapace in the form of a broad shield above the 

 trunk ; the compound eyes sessile and close together ; the antennules 

 and antennae much reduced ; and the trunk limbs numerous, the first 

 two pairs of them differing considerably from the rest. 



This order contains only the genera Apus and Lepidurus, which 

 differ in but minor features. Apus cancriformis (Fig. 231) is British, 

 but is now very rarely found in these islands. The head (Fig. 230) is 



Fig. 230. A ventral view of the head region of Lepidurus glacialis. From 

 Caiman, a.' antennule; a." antenna; gn. gnathobase; L. labrum (turned 

 forwards); /. paragnathum; md. mandible; mx.' maxillule; mx." maxilla. 



broad and depressed, flat below and arched above, and forms with the 

 carapace a horseshoe-shaped structure, which bears the eyes above and 

 the small antennules and antennae beneath, at some distance from the 

 sharp front edge. There is a dorsal organ, which is not used for fixation, 

 but no nuchal sense organ. From under the carapace the hinder part of 

 the trunk projects backwards, ending in two long, jointed caudal rami. 



