The infusoriform embryo has the form of a top or pear; the broad end is the head, the conical portion the tail. The head is non-ciliated, the tail ciliated. The former consists of two dorsally-placed bodies, the refractile organs, which are two modified ectoderm cells, and a single ventral organ, the urn. The urn consists of a cover formed by four modified ectoderm cells, of a floor and sides formed by two modified cells (? ectodermic), and of contents. The latter are four bodies, or at first cells, containing granules which sometimes show ciliary motion, and hence are probably spermatozoa (E. v. B.). They are produced by division of the nuclei of the four cells, which in their turn are said to be derived from the two cells modified to form the sides and floor of the urn. The tail of. the embryo is composed of ciliated cells. It swims head foremost. Two views are possible as to its nature: (i) That it spreads the race from one Cuttlefish to another. This view is supported by van Beneden's assertion that it lives longer in sea-water than a fully-formed Dicyemid, a fact denied however by Whitman. (2) That it is a male organism, a view supported by the observation that it sometimes occurs in a degenerative condition in the axial endoderm cell, and by its resemblance (not very close) to a male Orthonectid (supra).
The vermiform embryo is produced by the division of a germ-cell first into two, then into four, cells. Of the latter, one remains passive, and eventually grows into the axial endoderm cell of the adult; the other three divide and give origin to the ectoderm cells. At the time of birth the vermiform embryo resembles its parent, and contains germs, sometimes even embryoes. Birth, as in the case of the infusori-form embryo, takes place by simple perforation of the endoderm and ectoderm cells.
The Heterocyemidae differ from the Dicyemidae by the absence of a calotte, and in some minor points. There are two species, Conocyema polymorphus from Octopus vulgaris, and Microcyema Vespa from Sepia officinalis.
As to the first-named, the Nematogenous individual varies somewhat in shape. Four apical cells at one extremity (the head) tend to become verruciform, and sometimes throw out processes carrying stiff cilia. The remaining ectoderm consists of relatively few cells; their cilia are lost with age, and the cells themselves either flatten out and their limits become indistinguishable, or they may even be lost altogether. There is a single vacuolated endoderm cell. The vermiform embryo is formed as in Dicyema, but is conical in shape, the point of the cone being formed solely by the apical cells; the endoderm cell is spherical. The Rhombogen possess a more or less spherical endoderm cell, and an ectoderm consisting of a very few cells, non-ciliated and capable of throwing out pseudopodia; by their means the individuals are often united into colonies. The infusoriform embryo resembles that of Dicyema.
The adult Microcyema Vespa is cylindrical in shape, slightly enlarged at one end, and composed of a single endoderm cell covered by a thin non-ciliated cortical layer (=modified ectoderm?). The embryo in profile resembles closely a Wasp. Its anterior half is truncated, and consists of two ciliated ectoderm cells and a granular mass (? cell or cells), which bears long stout cilia; its posterior half is fusiform, and consists of two ciliated ectoderm cells inclosing a single endoderm cell.
It has been asked, is the establishment of a group, Mesozoa, for the reception of the Orthonectida and Rhombozoa justifiable? Do the organisms themselves really possess the exceedingly primitive structure assigned to them, or are traces of simplification of organisation observable?
It is self-evident that a great gap intervenes between the most highly differentiated unicellular Protozoon and the most lowly Metazoon, with ectoderm and endoderm, and mesoblast or mesoglaea. In every ontogeny a morula or blastula stage, and, except in certain parasites, a gastrula stage of some kind is to be traced; and it is of course to be concluded that they represent ancestral forms of development. None such are, however, known to exist as independent and perfected organisms, unless the Orthonectida and Rhombozoa are to be considered as modified gastrulae. The syncytium of Haeckel's Gastreadae (p. 809) is doubtless an ectoderm plus mesoglaea as it has proved to be in the calcareous sponges: and the disc-like organism, Trichoplax, (Schulze, Z. A. vi. 1883) has a cellular mesoglaea. They are therefore Metazoa.
The fact that the Mesozoa so-called are parasitic organisms naturally raises a presumption that they are in some way degenerate. Parasitism must in all cases be an acquired mode of life, and it invariably produces a change in the parasite, which is shown either by the simplification of some structures which have been evidently well developed in an ancestor, or by their total loss in the course of development. The gastrula of both Orthonectida and Rhombozoa is epibolic, a form which is usually not considered primitive. The most difficult point, however, is the question what character is to be assigned (I) to the 'intermediate endodermic cell masses' of the male Orthonectid and to the superficial layer of endoderm cells in the female respectively which give origin to muscular fibrils; and (2) to the two primitive germ-cells contained within the parent endoderm-cell of a Dicyemid, one at each pole? Are these endodermic derivatives to be considered as mesodermic cells originating, as mesodermic cells sometimes do in higher forms, from the endoderm; or are they to be compared with differentiated ectoderm cells which are still part and parcel of the ectoderm, as occurs with the epithelio-muscle-cells of some Coelenterata? The Mesozoa will or will not be considered as a rightly established section of the Animal Kingdom, according to which of these two alternatives be chosen.
The first points to an origin from some higher form; the second alternative naturally makes in favour of E. van Beneden's views. Further research may throw light on what it must be confessed is no easy problem.
The questions touched on above are discussed by the authorities cited below.
Orthonectida, Julin, Archives de Biologie, hi. 1882; Metschnikoff, Z. W. Z. xxxv. 1881.
Rhombozoa. Dicyemidaey Whitman, Mitth. Zool. Stat. Naples, iv. 1883; E. van Beneden, Bull. Ac. Roy. Belg. (2), xli. xlii. 1876; cf. D'Arcy Power, Q. J. M. xvii. 1876. Heterocyemidae, E. van Beneden, Archives de Biologie, iii. 1882.