ORTHONECTIDA.[114]—Two species of Orthonectids are fairly well known, _Rhopalura giardii_ Metschn. from _Amphiura squamata_, and _R. intoshii_ Metschn. from _Nemertes lacteus_. The latter appears to be very rare, the former occurring in 2 to 5 per cent of the number of hosts examined. The parasites occur in a granular "plasmodium," the nature of which is uncertain. Metschnikoff regards it as formed by the Orthonectids, and he considers that the cellular envelope, by which it is sometimes enclosed, is developed from the neighbouring tissue of the host. These granular, sometimes nucleated, plasmodial masses, which can perform active amoeboid movements in sea-water, occur attached to the ventral part of the body-cavity of _Amphiura_, and between the gut-branches and body-wall in _Nemertes_. Should these hosts be infected by great numbers of the Orthonectids, their sexual organs degenerate (as is the case with pond-snails attacked by sporocysts[115]), and it is possible that the remains of these organs may constitute the "plasmodia" (Braun). {95}_Rhopalura giardii_ is of distinct sexes. Either males or females are found in one _Amphiura_. Two kinds of females, flattened unsegmented, and cylindrical segmented forms, are known. They consist of a ciliated ectodermal layer enclosing an endodermal mass of eggs, between which is a fibrillar layer usually considered to be of a muscular nature. The cylindrical female gives rise to eggs which develop, probably exclusively, into males. The flattened female produces eggs from which females alone arise, though the origin of the two forms of this sex is not well ascertained. The males contain spermatozoa which fertilise the eggs of the cylindrical female, whereas the ova of the flat form probably develop parthenogenetically. [Illustration: FIG. 47.—_Rhopalura giardii_ Metschn. (from the brittle-star _Amphiura squamata_). ♂, Full-grown male (× 800); ♀_{1}, flattened form of female (× 510); ♀_{2}, cylindrical female (× 510). (After Julin.)] TRICHOPLAX.[116]—This anomalous animal has only been found in aquaria, originally in the marine aquarium at Graz by {96}Schulze. It has the appearance of a large, flattened, ciliated _Amoeba_ (1.5-3 mm. in diameter), but is distinguished by its structure. The upper surface is composed of a flattened epithelium. The lower surface is made up of cylindrical ciliated cells, which pass imperceptibly into the branched cells, embedded in a hyaline matrix, which compose the middle layer of the body. No distinct organs, and beyond simple fission, no mode of reproduction, have been observed. One species, _T. adhaerens_, is known, but has never been met with in a free state.