360 THE INVERTEBRATA 



Lepidocaris (Suborder Lipostraca), a minute, blind, freshwater form 

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

 survive (Euanostraca), 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 exo- 

 podite lateral, and the remaining 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 ; the trunk limbs numerous, the first two 



.fi nid. 



Fig. 241. 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. 



pairs of them differing considerably from the rest ; and slender, multi- 

 articulate caudal rami. 



This order contains only the genera Apus and Lepidurus^ which 

 differ in but minor features. Apus cancriformis (Fig. 242) is British, 

 but is now very rarely found in these islands. The head is broad and 

 depressed, fiat 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 fixa- 

 tion, but no nuchal sense organ. From under the carapace the hinder 

 part of the trunk projects backwards, ending in two long, jointed 

 caudal rami. The genital opening is on the i ith of the trunk somites. 

 Each of these bears a pair of limbs until the 13th (second of the ab- 

 domen) is reached, after which there are two to five pairs to a somite 



