The Rhombozoa are defined as follows by E. van Beneden: - 'Body never an-nulated; endoderm a single cell; no muscular fibres; the germs originate and develope within an axial cell. The male has the form of a top; certain non-ciliated cells at its anterior extremity inclose refractile bodies; females viviparous.' There are two families, Dicyemidae and Heterocyemidae.
Van Beneden considers that there are five genera of Dicyemidae, each genus confined to a genus of Cuttlefish, and a given species of each genus confined in turn to a given species of the corresponding genus of Cuttlefish 1. Whitman has however proved (1) that the same Cuttlefish may harbour two Dicyemids belonging to different genera; (2) that the same species of Dicyemid may be found in different species of the same genus of Cuttlefish, or even in different but closely allied genera (Sepia and Rossia); (3) that two closely allied species of Dicyemids occur in Cuttlefish belonging to different families, while two remotely allied species occur in two closely related genera of Cuttlefish (Sepia and Rossia). Whitman consequently proposes to range the known species of Dicyemids under two genera, Dicyema and Dicyemennea, characterised solely by anatomical differences. The largest species described by Whitman (Dicyema macrocephalum) attains a length of 5-7 mm.: the smallest (D. truncatum) only .50 - 75 mm.
Every Dicyemid consists of a head or calotte, and a more or less elongated body. The calotte is composed of two circlets of ectoderm cells, an anterior circlet of four propolar cells, a posterior of four metapolar cells in Dicyema, or of five, three dorsal and two ventral, in Dicyemennea. In the first-named the calotte is always symmetrical in the young animal; its cells arranged round an axis continuous with that of the body, hence orthotropal; but in the adult it becomes oblique and bent towards the ventral aspect, hence plagiotropal. The calotte is followed by two parapolar ectoderm cells, always lateral in position, one on the right, the other on the left. The remaining ectoderm cells cover the body: their number varies in the different species. The caudal extremity is formed by two cells, of which one is usually dorsal, the other ventral. The ectoderm cells are ciliated on their external faces, are uninucleate, and contain granules of different kinds. Certain of them, always few in number, often have their centres or posterior extremities swollen into a wart, hence verruciform cells; and in these warts are collected numbers of refractile globules. The edges of the cells fit one with another.
The axis of the body is occupied by a single huge endoderm cell which extends from the propolar cells to the caudal extremity, covered at every point by the ectoderm. It is in shape a cylinder pointed at each end. Its protoplasm is largely vacuolated; the vacuoles contain a hyaline liquid, immiscible with water. Its nucleus is large, with a tough membrane, and contains a nuclear network.
Every Dicyemid, according to Whitman, is either monogenic or diphygenic. The first-named produces only vermiform embryoes, and is hence a primary Nema-togen. The second produces (1) infusoriform embryoes (or males, E. v. B.), and then (2) vermiform embryoes; it is consequently first a Rhombogen (E. v. B.). and next a secondary Nematogen2. On this view the Rhombogen and secondary Nematogen are successive phases in the life-history of one and the same individual, but it is not known whether the monogenic and diphygenic individuals are or are not different forms. The distinction between them, which was not recognised by E. van Beneden, depends solely on the different modes in which their germs de-velope. The primary Nematogen is most common in young Cuttlefish, the Rhombogen in adult.
1Van Beneden's names are as follows: Dicyema, confined to the Cephalopod genus Octopus; Dicyemella to Eledone; Dicyemina to Sepia; and Dicyemopsis to Sepiola. See the historical section in Whitman's paper, Mitth. Zool. Stat. Naples, iv. 1883, p. 3.
2Rhombogen = individual producing infusoriform embryoes, Nematogen = individual producing vermiform embryoes, accordirg to E. van Beneden's nomenclature.
In a young Dicyemid the axial endoderm cell contains in addition to its nucleus several germ-cells. The first and second germ-cells are derived by division of the nucleus of the axial endoderm cell and a separation of a part of its protoplasm round each of the nuclei thus formed. The remaining germ-cells are derived by binary fission of the two first formed. In a Rhombogen each germ-cell present developes as follows. First of all it divides: one half consists of a nucleus plus a very small portion of protoplasm, if any at all. This half is clear, and remains in statu quo as a 'paranucleus.' The other half consists of nucleus and protoplasm, and it proceeds to divide into a number of cells surrounding a single central cell. The whole group is called by Whitman Infusorigen, a term used by E. van Beneden as synonymous with Rhombogen. The central cell of the group, or germogen, produces fresh cells endogenously, i. e. by division of the nucleus within the germogen. The cells of the group are set free, one or two at a time; they are large with large nuclei. The endogenously produced cells are similarly detached. All of them de-velope into infusoriform embryoes.
Finally, the germogen gives rise to a number of small cells with small nuclei, which lie loose, i. e. not coherent in a group, multiply by division, and fill the axial endoderm cell in great measure. From them originate the vermiform embryoes, but the development of the latter rarely occurs before all the infusoriform embryoes have escaped. The germogen cell, exhausted by the work of fission, is finally reduced to a 'residual nucleus.'
The axial endoderm cell of a primary Nematogen contains only its own nucleus and germ-cells developing into vermiform embryoes; of a Rhombogen its own nucleus + one or more paranuclei equal in number to the Infusorigens present; of a secondary Nematogen, its own nucleus + a number of paranuclei + a number of residual nuclei equal in number to the Infusorigens that have been resolved, + many germ-cells developing or about to develope into vermiform embryoes. The maximum number of Infusorigens produced is generally eight. Whitman is inclined to believe that an Infusorigen is an individual equivalent to the Gastrula of the vermiform embryo.