The animals composing this group are minute and parasitic, and are composed of a small number of cells. They may be divided into two families: the Dicyemidae, which occur exclusively in the kidneys of certain Cephalopods (cuttle-fish); and the Orthonectidae, which live in the brittle-star Amphiura squamata, the Nemertine Nemertes lacteus, or the Polyclad Leptoplana tremellaris. In addition to the undoubted Mesozoa, certain anomalous forms—Trichoplax adhaerens and Salinella salve—may be referred to this group.
Fig. 46.—Dicyemennea eledones Wag., from the kidney of Eledone moschata. A, Full-grown Rhombogen with infusoriform embryos (emb); B, one of the latter developing; C, fully formed; D, calotte, composed of the upper nine cells shown in the figure. (After Ed. van Beneden and Whitman.) emb, Infusoriform embryo; g, part of endoderm-cell where formation of these embryos is rapidly proceeding; n.ect, nucleus of ectoderm-cell; n.end, nucleus of endoderm-cell; p, "calotte."
Dicyemidae.—If the kidney of Eledone moschata, a Cephalopod common on our south-western shores, be opened, a number of fine, yellowish, hair-like filaments may be seen attached at one end to its inner surface, floating in the fluid contained in the renal cavity. These may be Dicyemennea eledones Wag., although another form, Dicyema moschatum Whit., also occurs in the same host. D. eledones (Fig. 46) is 7 to 9 mm. long, transparent, and is composed of one large inner cell with a simple nucleus (Fig. 46, n.end), and of an outer layer of ciliated cells, nine of which form the "calotte" or pole by which the animal is attached. Within the former (endodermal) cell the formation of urn-shaped "infusoriform embryos" takes place (B and C), the fate of which is not known, but they are possibly the males. The individual which produces these larvae is called a "Rhombogen." Other individuals which produce a more elongated larva ("vermiform larva," Fig. 45) are called "Nematogens," and Whitman has described a third kind, which produce first infusoriform, and then vermiform, larvae (Secondary Nematogens).[[113]]
The occurrence of the known species of Dicyemids (a group which has not been investigated on our coasts) is as follows:—
| Species. | Host. |
| Dicyema typus van Ben. | Octopus vulgaris. |
| D. clausianum van Ben. | O. macropus. |
| D. microcephalum Whit. | O. de Filippi. |
| D. moschatum Whit. | Eledone moschata. |
| D. macrocephalum van Ben. | Sepiola rondeletii. |
| D. truncatum Whit. | Rossia macrosoma, Sepia elegans, S. officinalis. |
| D. schultzianum van Ben. | S. biseralis, Octopus vulgaris. |
| Dicyemennea eledones Wag. | Eledone moschata, E. aldrovandi. |
| D. mülleri Clap. | E. cirrosa. |
| D. gracile Wag. | Sepia officinalis. |
| Conocyema polymorphum van Ben. | S. officinalis, Octopus vulgaris. |
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).
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.
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.)