FIG. 36.—Diagrammatic transverse section through the distal part of the siphon of _Rhynchomyzon purpurocinctum_ (Asterocheridae). _Md_, mandible. (After Giesbrecht.) ] =Fam. 6. Asterocheridae.=[47]—These forms retain the power of swimming actively, and are very little modified in outward appearance by their parasitic mode of life (Fig. 35), though they possess a true siphon in which the styliform mandibles work. The siphon is formed by the upper and lower lips, which are produced into a tube with three longitudinal ridges; in the outer grooves are the mandibles, while the inner groove forms the sucking siphon (see transverse section, Fig. 36). In _Ratania_, however, there is no siphon. The first antennae possess a great number of joints, and may be geniculated in the male (_Cancerilla_). The members of this family live as ectoparasites on various species of Echinoderms, Sponges, and Ascidians, but they frequently change their hosts, and it appears that one and the same species may indifferently suck the juices of very various animals, and even of Algae. _Cancerilla tubulata_, however, appears to live only on the Brittle Starfish, _Amphiura squamata_. =Fam. 7. Dichelestiidae.=—The males and females are similarly parasitic, and the body in both is highly deformed, the segmentation being suppressed and the thoracic limbs being produced into formless fleshy lobes; they are placed among the Ampharthrandria owing to sexual differences in the form of the first antennae. There is a well-developed siphon in which the mandibular stylets work, except in _Lamproglena_, parasitic on the gills of Cyprinoid fishes; the succeeding mouth-parts are prehensile. The majority of the species are parasitic on the gills of various fish (_Dichelestium_ on the Sturgeon, _Lernanthropus_[48] on _Labrax lupus_, _Serranus scriba_, etc.), but Steuer[49] has recently described a Dichelestiid (_Mytilicola_) from the gut of _Mytilus galloprovincialis_ off Trieste. This animal and _Lernanthropus_ are unique among Crustacea through the possession of a completely closed blood-vascular system which contains a red fluid; the older observers believed this fluid to contain haemoglobin, but Steuer, as the result of careful analysis, denies this. The parasite on the gills of the Lobster, _Nicothoe astaci_, possibly belongs here. The inclusion of _Nicothoe_ and the Dichelestiidae among the Ampharthrandria rests on a somewhat slender basis; this basis is afforded by the fact that none of the parasitic Isokerandria have more than seven joints in the first antennae, whereas _Nicothoe_ and some of the Dichelestiidae[50] have more numerous joints. In most of the Dichelestiidae, however, the number of joints is less than seven and practically equal in the two sexes. TRIBE II. ISOKERANDRIA. The first antennae are short, similar in the two sexes, and are never used by the male as clasping organs. This function may be subserved by the second maxillae.