Fig. [91].—Ventral view of male Bopyrus fougerouxi, × 30. A, 1st and 2nd antennae; T, 8th (last) thoracic appendage. (After Bonnier.)
The adult female condition, which is assumed after the Bopyrid stage is passed through, is illustrated in Fig. [92]. The body acquires a remarkable asymmetry, due to the unequal pressure exerted by the walls of the gill-chamber. The antennae and mandibles (Fig. [92], B) are entirely covered up by the largely expanded maxillipedes; maxillae are, as usual, entirely absent. Very large lamellae grow out from the bases of the thoracic limbs to form a brood-pouch, and in this manner the adult condition is attained.
The final complication in the life-histories of these Isopoda is reached by the family Entoniscidae, which are parasitic when adult inside the thoracic cavity of Brachyura and Paguridae. The cephalothorax of a Carcinus maenas, which contains an adult Portunion maenadis (P), is shown in Fig. [93]. The parasite is of a reddish colour when alive.
Fig. [92].—Bopyrus fougerouxi. A, Ventral view of female carrying a male (M) between her abdominal appendages, × 8; B, ventral view of part of head of female, the maxillipedes and the left mandible having been removed. A.1, A.2, 1st and 2nd antennae; M, male; Mn, right mandible; Mx, left maxillipede; O, oostegite; T, left 4th thoracic appendage or 3rd leg. (After Bonnier.)
Fig. [93].—Cephalothorax of Carcinus maenas, seen from the ventral side, containing a parasitic Portunion maenadis (P), × ½. (After Bonnier.)
The Entoniscidae pass through a free living Epicaridian and Cryptoniscus stage, and become adult males in the Bopyrus stage. It is stated, however, by Giard and Bonnier[[109]] that these individuals, which actually function as males, never grow up into adult females, though all the adult females have passed through a male stage in which the male genital ducts are not formed. The hermaphroditism, therefore, in these animals at any rate is absolutely useless from a reproductive point of view, and this justifies our looking for some other explanation of it, such as was suggested on p. 105.
Fig. [94].—Portunion maenadis, ♀:—A, Young, × 10; B, older, × 5; C, adult, before the eggs are laid, × 3. A, 2nd antenna; Ab, abdomen; B, anterior lobe of brood-pouch; B′, its lateral lobe; H, head; 1, 2, 1st and 2nd incubatory lamellae (oostegites). (After Giard and Bonnier.)