136 CRUSTACEA—PERACARIDA CHAP. 
the abdominal segments (40). In the final stage (C) the inecu- 
batory lamellae have further increased in size, and constitute 
the main bulk of the body; the enormous mass of eggs is passed 
into the incubatory pouch, and all that remains of the rest of the 
body is the small head (H) and the abdomen (4d), furnished 
with its branchiae. Communication with the external world is 
kept up through an aperture which leads from the brood-pouch 
into the gill-chamber of the host, and through this aperture the 
young are hatched out when they are developed sufficiently. 
The presence of these parasites, although they are never in 
actual contact with the internal organs of the crab, calls forth 
the same phenomenon of parasitic castration as was observed in 
the Rhizocephala. A remarkable association is also found to 
exist between the Entoniscidae and Rhizocephala, of such a kind 
that, on the whole, a crab infested with a Rhizocephalan is more 
likely to harbour an Entoniscid than one without. The explana- 
tion of this association 1s probably that a crab with a Sacculina 
inside it is prevented from moulting as often as an uninfected 
crab, and, in consequence, the larval stages of the Entoniscid in 
the crab’s gill-chamber are more safely passed through. 
Sub-Order 7. Phreatoicidea.' 
The members of this sub-order, although agreeing with the 
Isopoda in the essentials of their anatomy, resemble the Amphi- 
poda in being rather laterally compressed, and in having the 
hand of the first free thoracic limb enlarged and subchelate. 
The abdomen is greatly produced laterally by expansions of the 
segments. In fact, the shape of the body and of the limbs is 
very Amphipodan.—Phreatoicus from New Zealand, Southern 
Australia, and Tasmania. Phreatoicopsis, a very large form from 
Gippsland, Victoria. Only one family exists, Phreatoicidae. 
Order IV. Amphipoda. 
In this order the body is flattened laterally, the heart is 
anterior in position, and the branchial organs are attached to 
the thoracic limbs. 
There are three well-defined sub-orders, (i.) the Crevettina, in- 
1 Chilton, Trans. Linn. Soc. vi., 1894, p. 185. 
* Spenser and Hall, Proc. Roy. Soc. Victoria, ix. p. 12. 
