136 CRUSTACEA PERACARIDA CHAP. 



the abdominal segments (Ab}. In the final stage (C) the incu- 

 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 (AV), 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 is 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. 1 



The members of this sub-order, although agreeing with the 

 Isopoda in the essentials of their anatomy, resemble the Amphi- 

 poda in be'ing 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, iu- 



1 Chilton, Trans. Linn. Soc. vi., 1894, p. 185. 

 '* Spenser and Hall, Proc. Roy. Soc. Victoria, ix. p. 12. 



