The animal appears to be in an arrested state of development, and so retains some of the characteristics of the Cypris larvae, but it is very doubtful how far these characters can be considered primitive.
Other forms are Dendrogaster astericola on Echinoderms, and Synagoga mira on the “Black Coral,” Parantipathes larix, at Naples.
Sub-Order 5. Apoda.
Fig. [65].—Proteolepas bivincta, × 26. A, Antennae; a, b, 1st and 2nd abdominal segments; O, ovary; P, penis; T, telson; 1–8, thoracic segments. (After Darwin.)
Darwin described a small hermaphrodite parasite in the mantle chamber of Alepas cornuta from Saint Vincent, West Indies, which he named Proteolepas bivincta.
The body (Fig. [65]) is distinctly segmented into eleven somites, the last three of which are supposed to belong to the abdomen; there are no appendages except the antennae by which fixation is effected. The mouth-parts are of normal constitution.
This animal has not been found again since Darwin’s discovery, but Hansen[[71]] describes a number of peculiar Nauplius larvae taken in the plankton of various regions, which he argues probably belong to members of this group. A wide field of work is offered in attempting to find the adults into which various larvae grow.
Sub-Order 6. Rhizocephala.[[72]]
These remarkable animals are Cirripedes which have taken to living parasitically on various kinds of Crustacea; the majority infest species of Decapoda, e.g. Peltogaster on Hermit-Crabs, Sacculina on a number of Brachyura, Sylon on Shrimps, Lernaeodiscus on Galathea; but one genus, Duplorbis, has been found in the marsupium of the Isopod Calathura brachiata from Greenland. Most of the species are solitary, but a few, e.g. Peltogaster sulcatus, are social. In the adult state the body consists of two portions: a soft bag-like structure, external to the host, carrying the reproductive, nervous, and muscular organs, and attached to some part of the host’s abdomen by means of a chitinous ring; and a system of branching roots inside the host’s body, which spring from the ring of attachment and supply the external body with nourishment.