VALVIFERA ASELLOTA 



127 



of the mouth-parts, while the maxillipedes undergo an enlarge- 

 ment, and are used for causing a current through the brood- 

 chamber. 



Sub-Order 3. Valvifera. 



The Valvifera, illustrated by the Idotheidae and Arcturidae, 

 are characterised by the uropods being turned back and expanded 

 to form folding doors covering up the delicate pleopods, which are 

 mostly respiratory 

 in function, though 

 the anterior pairs 

 may serve as swim- 

 ming organs. Arc- 

 turns is a typically 

 deep sea genus, 

 many species, re- 

 markably furnished 

 with spiny processes, 

 having been taken 

 by the Challenger in 

 the southern hemi- 

 sphere. The Ido- 

 theidae are more lit- 

 toral forms, several 

 species of Idothea 

 being commonly 

 met with off the 

 British coasts, oc- 

 casionally penetrat- 

 ing into brackish or 

 even fresh water. 



4. Fie. 83. Alunnopsis typica (Munnopsidae), /$ , x 2. A, 



~ uc ^ an ^ eniia ; Ab, abdomen ; T, 5th thoracic appendage 

 or 4th leg. (After Sars.) 



In this group 



the abdominal segments are fused dorsal ly to form a shield-like 

 caudal region ; the pleopods are respiratory in function and 

 reduced in numbers, the first pair being often expanded and 

 produced backwards to form an operculum covering the rest. 

 Several of the Asellota are fresh -water, Asellus aquatint* 



