ARTHROPODA 293 
from the system, and although they open close to its end, they 
belong to the mid gut and not to the proctodaeum. 
The abdomen is reduced to a limbless process in the 
LAEMODIPODA, a group which includes Cyamus ceti parasitic on 
Fig. 169.—Gammarus neglectus. Section through the 
third thoracic segment. From Leuckart and Nitsche, 
after G. O. Sars. 
— 
. The heart. ®& : 
ae he alimentar?” canal. 
. The ovary. 
. The hepatic diverticula. 
The nervous system. 
The epimeron. 
“STO OP OO LO 
. The thoracic appendage bearing— 
. The lamina forming the brood-pouch (oostegite), and 
. The branchiae. 
co CO 
the skin of whales, and in the family CAPRELLIDAE which 
comprises curious elongated thin animals of a very grotesque 
appearance, mostly found living amonest Polyzoa and Hydroids. 
The family PHRONIMIDAE includes some forms which have an 
enlarged head; the female Phronimea is a good deal larger than 
the male, and is usually found swimming about in a barrel- 
shaped investment formed by the transparent. colonies of the 
compound Ascidian Pyrosoma; other allied forms live in 
jelly-fish. 
Sub-order 2. ISOPODA. 
CHARACTERISTICS. — Arthrostraca with usually a dorso-ventrally 
depressed body ; thorax of six or seven free segments ; abdomen 
often reduced, it bears lamelliform appendages, the endo- 
podites of some of which function as gills. 
A very typical example of an Isopod is presented by the 
little brownish-gray animal <Asel/us aquaticus, which inhabits 
lakes, large ponds, and marshes. Its body is flattened, and 
composed of three regions—the head, the thorax, and the abdo- 
men; of these the thorax is far the largest, and consists of 
seven free segments and one fused with the head. The abdom- 
inal segments have all fused together with the exception of 
the first, which is very small, and bears rudimentary appendages. 
