ISOPODA. 305 



in the general segmentation of the body, being still distinguishable 

 from the latter by the relatively large size of the head and the eyes, 

 the incomplete segmentation and the non-existence of setae on the 

 limbs, and above all by the absence of the last pair of thoracic limbs. 

 This young form passes gradually through several moults into the 

 adult stage. Schiodte and Meinert (No. 175) distinguished in 

 the Aegidae (and Gymothoidae generally) three consecutive larval 

 stages, the youngest of Avhich, found still in the brood-cavity of 

 the mother, is marked by the absence of setae on the limbs and 

 the telson. The second free-swimming stage has already developed 

 setae, and during the third stage tlie limb-rudiments develop on the 

 last thoracic segment. In the Gymothuidae we meet with indications 

 of degeneration owing to the parasitic life adopted by these forms ; 

 these even appear during metamorphosis, finding expression in the 

 shortening of tlie antennae and the changing of the thoracic limbs 

 into adhesive organs. 



In individual cases, these changes lead, tlirongli the reductions caused by 

 parasitism, to a much more distinctly marked metamorphosis, the sexual 

 heteromorphism also being more accentuated than in the free-living Isopoda. 

 This is the case in the families of the Anceidae, Bopyridae, and Entoniscidae. 



In the genus Gnathia, the female form (Praniza) is very different from the 

 sexually mature male (Anceits). In the female, the head is small and triangular, 

 and three of the posterior thoracic segments fuse to form a swollen region 

 (brood-chamber) ; in the male, the head is broad and square, and the seizing 

 pincers are branched like the horns of the stag-beetle. The young forms of 

 this family show the elongate Praniza type, but from the very earliest stages 

 there are indications of sexual dimorphism, the larvae which will become 

 females already showing signs of the fusion of three thoracic segments, which, 

 in the young males, are quite distinct from one another. These Praniza-like 

 larvae lead a parasitic life (on fishes). Consequently, month-parts which are 

 adapted for yiiercing and for suction project forward under a large upper lip. The 

 mandibles and maxillae are adapted for piercing ; they are palpless, and pointed 

 like stylets, and the mandible and second maxilla are toothed at their ends. A 

 pair of maxillipedes follows, the segment carrying them being fused with the head. 

 This anterior maxillipede, which is somewhat long, forms a kind of low^er lip, 

 while the second gnathojiod ends in an anchoring hook. The five following thoracic 

 segments (the third to the seventh) carry live pairs of limbs clianged into 

 anchoring organs, the fifth to seventh being fused in the female.* The eighth 

 thoracic segment is retained in a very rudimentary condition, and is followed by 

 a well-segmented abdomen furnished with biramose jtleopoda used for swimming. 

 During the transformation into the adult form, the upper lip and the maxillae 

 are lost, while the maxillipedes undergo important alteration. They become 

 lamellate organs for promoting circulation of the surrounding water. In the 



* [According to Stebhing {A History of Crustacea, London, 1893) it is the 

 fourth, fiftii, and often the sixth thoracic segments which fuse aud form the 

 brood-chamber. — Ed.] 



X 



