Fig. [81].—Apseudes spinosus, ♂, × 15. A, 1st antenna; Ab, 6th abdominal appendage; T, 2nd thoracic appendage. (After Sars.)
Both families, of which the Apseudidae contain the larger forms, sometimes attaining to an inch in length, are littoral in habit, or occur in sand and ooze at considerable depths, many of the genera being blind. Many Tanaids (e.g. Leptochelia, Tanais, Heterotanais, etc.) live in the algal growths of the littoral zone, and being highly heliotropic they are easy to collect if a basinful of algae is placed in a strong light. The females carry the eggs about with them in a brood-pouch formed, as is usual in the Peracarida, by lamellae produced from the bases of the thoracic limbs. The males on coming to maturity do not appear to grow any more, or to take food, their mouth-parts frequently degenerating and the alimentary canal being devoid of food. They are thus in the position of insects which do not moult after coming to maturity; and, as in Insects, the males are apt to show a kind of high and low dimorphism—certain of the males being small with secondary sexual characters little different from those of the females, while others are large with these characters highly developed. Fritz Müller, in his Facts for Darwin, observes that in a Brazilian species of Leptochelia, apparently identical with the European L. dubia, the males occur under two totally distinct forms—one in which the chelae are greatly developed, and another in which the chelae resemble those of the female, but the antennae in this form are provided with far longer and more numerous sensory hairs than in the first form. Müller suggested that these two varieties were produced by natural selection, the characters of the one form compensating for the absence of the characters of the other. A general consideration of the sexual dimorphism in the Tanaidae[[99]] lends some support to this view, since the smaller species with feeble chelae do appear to be compensated by a greater development of sensory hairs on the antennae, but the specific differences are so difficult to appreciate in the Tanaidae that it is possible that the two forms of the male in Müller’s supposed single species really belonged to two separate species.
Sub-Order 2. Flabellifera.
The Flabellifera include a number of rather heterogeneous families which resemble one another, however, in the uropods being lateral and not terminal, and being expanded together with the telson to form a caudal fan for swimming. The pleopods are sometimes natatory and sometimes branchial in function. Some of the families are parasitic or semi-parasitic in habit.
Fam. 1. Anthuridae.—These are elongated cylindrical creatures found in mud and among weeds upon the sea-bottom; their mouth-parts are evidently intended for piercing and sucking, but whether they are parasitic at certain periods on other animals is not exactly known. Anthura, Paranthura, Cruregens.
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.
Fam. 3. Cymothoidae.[[101]]—This is a group of parasites more completely parasitic than the foregoing, but their outer organisation does not differ greatly from an ordinary Isopodan form. A great many very similar species are known which infest the gill-chambers, mouths, and skin of various fishes. The chief interest that attaches to them is found in the fact that a number of them, and perhaps all, are hermaphrodite, each individual acting as a male when free-swimming and young, and then subsequently settling down and becoming female. This condition is exactly the same as that occurring universally in the great group of parasitic Isopoda, the Epicarida, to be considered later. There is no evidence that the Cymothoidae are phyletically related to the Epicarida, so that the similar sexual organisation appears to be due to convergence resulting from similar conditions of life. The general question of hermaphroditism in the Crustacea has been shortly discussed on pp. 105–106. Cymothoa.
Fam. 4. Cirolanidae.—In this family is placed the largest Isopod known—the deep-sea Bathynomus giganteus, found in the Gulf of Mexico and the Indian Ocean, sometimes measuring a foot long by four inches broad. A common small littoral form is Cirolana.