Vv VALVIFERA——ASELLOTA 27) 
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- 
turus is a typically 
deep sea genus, 
many species, re- 
markably furnished 
with spiny processes, \ 
having been taken \ 
by the Challenger in \ 
the southern hemi- Ane \ 
sphere. The  Ido- 
theidae are more lit- 
toral forms, several 
species of /dothea 
being commonly (5 oy, 
met with off the Nepal hay 
British coasts, oc- Rice 
casionally penetrat- 
ing into brackish or 
even fresh water. NS 
afi 
Sub-Order 4. Fic. 83.—Munnopsis typica (Munnopsidae), 6, x 2. A, 
Asellota. 2nd antenna; Ab, abdomen ; 7’, 5th thoracic appendage 
or 4th leg. (After Sars.) 
In this group 
the abdominal segments are fused dorsally 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 aquaticus 
