FIG. 82.—_Gnathia maxillaris_. =A=, Segmented larva, × 10; =B=, Praniza larva, × 5; =C=, gravid female, × 5; =D=, male, × 5. ] =Fam. 2. Gnathiidae.=[100]—These forms appear to be related to the Anthuridae; they are ectoparasitic on various kinds of fish during larval life, but on assuming the adult state they do not feed any more, subsisting merely on the nourishment amassed during the larval periods. The larvae themselves are continually leaving their hosts, and can be taken in great numbers living freely among weeds on the sea-bottom. The larvae, together with the adults of _Gnathia maxillaris_, are extremely abundant among the roots of the sea-weed _Poseidonia cavolinii_ in the Bay of Naples. The young larvae hatch out from the body of the female in the state shown in Fig. 82, A. This minute larva fixes upon a fish, and after a time it is transformed into the so-called Praniza larva (B), in which the gut is so distended with the fluid sucked from the host that the segmentation in the hind part of the thorax is entirely lost. When this larva moults it may, however, reacquire temporarily its segmentation. After a certain period of this parasitic mode of life the Praniza finally abandons its host, and becomes transformed into the adult male or female. This may take place at very different stages in the growth of the larva, the range of variation in size of the adults being 1–8 mm., and it must be remembered that when once the adult condition is assumed growth entirely ceases. What it is that determines the stage of growth in each individual when it shall be transformed into the adult is not known. The males and females differ from one another so extraordinarily that it was for long denied that they were both derived from the Praniza larvae. This is nevertheless the case. The change from the Praniza to the female (Fig. 82, C) is not very great. The ovary absorbs all the nourishment in the gut and comes to occupy the whole of the body, all the other organs degenerating, including the alimentary canal and mouth-parts. Indeed, only the limbs with their muscles and the nervous system remain. The change to the male (D) is more radical. The food is here stored in the liver, which increases in the male just as the ovary does in the female. The segmentation is reacquired, and the massive square head is formed from the hinder part of the head in the Praniza, the anterior portion with its stylet-like appendages being thrown away. The powerful nippers of the male are not formed inside the cases of the old styliform mandibles, but are independent and possibly not homologous organs. The meaning of the marked sexual dimorphism and the use of the males’ nippers are not in the least known, though the animals are easy to keep under observation. In captivity the males never take the slightest notice of either larval or adult females.