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The most completely parasitic members of the Cymothoidæ are found in the subfamily Cymothoinæ, including the typical genus Cymothoa ([Plate XXIX].) and many others. The adult animals are found clinging to the skin of fishes, the legs being provided with strong hook-like claws that give them a very firm hold. Some species, especially common on Flying-fishes, cling to the tongue of the fish, and almost prevent it from closing its mouth. When young, the Cymothoinæ swim freely, and the shape of the body is not unlike that of the Cirolaninæ; but after they have settled on a host the body often becomes distorted and unsymmetrical. A still more remarkable change occurs in the reproductive organs in some, if not in all members of this subfamily. Each individual, when it first attaches itself to a host, presents the characters of the male sex. Later it becomes a female, develops a brood-pouch, and produces eggs. The animals are, in fact, hermaphrodite; but it is to be noted that the hermaphroditism is of a different kind from that presented by the Cirripedia, since the organs of the two sexes are successively, not simultaneously, developed. Where, as in this case, the male phase comes first in the life-history of the individual, the condition is known as "protandrous" hermaphroditism.

Another large group of parasitic Isopods is the suborder Epicaridea, all the species of which are parasitic on other Crustacea. It is not uncommon to find specimens of the common Prawn (Leander serratus) which have a large swelling on one side of the carapace. If the lower edge of the carapace be raised, it will be seen that this swelling is due to the presence in the gill cavity of an Isopod parasite (Bopyrus squillarum). A closely similar form, found on Prawns of the genus Spirontocaris, is Bopyroides hippolytes, represented in [Fig. 71]. Other allied species are found on Hermit Crabs and other Decapods. When extracted, the parasite is seen to have a flat and curiously distorted body, with extremely short legs ending in hooked claws. The under-side is generally occupied by a relatively enormous mass of eggs, which is only partly covered in by the small brood-plates. The mouth parts form a short piercing beak with which the parasite sucks the blood of its host. On the under-side of the abdomen may usually be found the minute male, attached, like a secondary parasite, to the body of the female.

Fig. 71—A, Front Part of Body of a Prawn (Spirontocaris polaris), from Above, showing on the Right Side a Swelling of the Carapace caused by the Presence of the Parasite Bopyroides hippolytes in the Gill Chamber; B, the Female Parasite extracted and further enlarged; C, the Male Parasite on Same Scale as the Female. (After Sars.)

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The species of Epicaridea are very numerous, and they infest Crustacea belonging to nearly all the chief groups of the class, a few even being parasitic on other Epicaridea. Many of them differ greatly from the Bopyrus just described, and in some cases it would be impossible to guess from the structure of the adult animals that they were Isopoda, or even Crustacea at all. The life-history is not yet completely known. When hatched from the egg, the free-swimming larvæ have a short and broad body, and, as in other Isopod larvæ, have only six instead of seven pairs of legs. A later larval stage, just before attachment to the final host, has a long narrow body and the full number of legs. It has lately been shown, however, that, in all probability, between these two free-swimming stages there intervenes a stage in which the larvæ is temporarily parasitic on certain Copepoda. Further, some of the Epicaridea, like the Cymothoinæ described above, are protandrous hermaphrodites, developing the male organs when in the last larval stage, and passing into the female phase after they have become attached to the host. In Bopyrus and many other genera, however, there is no evidence that the males ever develop into females.