Multicellular animals, in which there is only an ectoderm and endoderm or epi- and hypo-blast.

The ectoderm is composed of a single layer of cells, totally or partially ciliated, the endoderm of a single cell or of several cells. There is no mesoglaea nor meso-blast. The sexes are separate, and the sexual products originate from the endoderm. E. van Beneden adds that 'there are two forms of female, one producing females alone, the other males;' and that 'the Mesozoa actually known are all parasites.' The first of these two assertions does not apply to the Rhombozoa, according to Whitman; the second points to the possibility of the known Mesozoa being degenerate Metazoa.

There are two subdivisions of Mesozoa, according to the Belgian naturalist, the Orthonectida and Rhombozoa.

The Orthonectida are defined as follows by E. van Beneden: 'Body composed of several annuli; endoderm formed of several cells, some of which are epithelio-muscle cells, and give origin to muscular fibres, whilst the remainder are converted into sexual products. The male is elongated, annulated, and one of its anterior rings is papillate. Females oviparous' and, it may be added, dimorphic.

There is a single genus, Rhopa/ura, with two species; one, Rh. Giardi, parasitic in the genital bursae of the Ophiurid Amphiura squamata (=Ophiocoma neglecta); the other, Rh. Intoshii, in the Nemertean Lineus lacteus. Both species are therefore marine.

Rh. Giardi has been recently investigated by Julin. The males and females are usually to be found in different individuals of the Ophiurid. The parasites cause an atrophy of their host's genitalia. The males are contained in saccules, apparently produced by the host, in which they swim freely, and are, when adult, about 104 mm. long. Their body is fusiform and divided usually by five furrows into six annuli. Each annulus is composed of a single layer of ectoderm cells. The first or head is constituted by 4 or 8 cells: the second of five rows of small cells, the so-called papillae, all of which are non-ciliate. The third annulus is usually the longest, and contains one row of cells. The last or caudal annulus has two rows of 4 cells, and when the rows are very distinct, it may be considered as split into two annuli. The cilia of the head are directed forwards; of the third and following annuli backwards. The caudal cilia are long and stout. At the level of the third annulus is contained the testis formed from the central endoderm cells. The spermatozoa possess a head and tail. Fibrils stretch from either end of the body beneath the ectoderm over the testis; nuclei have been detected in them.

The ripe sperm ruptures the testis: the fibrils (supra) thereupon become collected into bundles, the ectoderm disorganised, its cells swelling and separating, while the sperm escapes into the water. The females occur in two forms, one cylindrical, which produces only male embryoes, the other flattened but pointed at each end, from which only female embryoes originate. Both forms occur side by side. The first-mentioned is -280 mm. long. It has eight annuli: the second annulus which, like the third and fifth, contains only one row of cells, is non-ciliate in the adult. The remaining annuli vary in the number of their rows of cells. There is a sub-ectodermic fibrillar layer. The central mass consists of ova, polyhedric from mutual pressure, but free from one another when they are discharged into the water by the rupture of the body in the furrow behind the second annulus. The flattened female is -250 mm. long. It is completely ciliated, and the cilia of the anterior and posterior regions are directed as in the male and cylindrical female. Furrows are usually wanting, and the ectoderm cells are not distinguishable during life.

Sub-ectodermic flbrillae are present, and near the anterior extremity of the body there is a uni-nucleate granular mass (the sub-polar cells of Metschnikoff) projecting inwards from the ectoderm. The ova are held together by an intervening granular substance. This female appears to break into segments, the Plasmodiumschlauche of MetschnikofT, which swim about and contain the developing and developed female offspring.

The male ovum divides into two blastomeres; one large, the endoderm cell; the other small, which divides in turn repeatedly, grows round the endoderm cell, and forms the ectoderm. The endoderm cell divides into three, a central and two polar, one anterior, the other posterior. The two polar cells divide in their turn and form small 'intermediate cell-masses' from which the muscle-fibrils are derived. The central cell also divides and gives origin to the sperm. The youngest female observed consists of a mass of endoderm cells, at first partially, then wholly surrounded by ectoderm cells. The muscle-fibrils are produced from a superficial layer of endoderm cells. It may be observed that in both sexes there is an epibolic Gas-trula. The young Rhopalura is at first entirely ciliated, and the cilia are subsequently lost from the second annulus. The five rows of papillae in the male are produced by the division of two rows of ectoderm cells.

Rh. Intoshii is very similar to Rh. Giardi, but it is smaller. The male has no papillate annulus, nor has either sex a non-ciliated annulus. The cilia of the head are directed forwards, of the other annuli backwards. This species is found in saccules (Plasmodiumschlauche) between the body-walls and digestive tract of its Nemertean host. They are probably formed by the host, in which the genitalia, owing to the presence of the parasite, appear either to remain undeveloped, or to atrophy. The saccules may contain only males, or females, or both together, and in variable numbers. Metschnikoff does not seem to have observed a second form of female. It is uncertain whether Rh. Intoshii is or is not identical with the Ortho-nectidan observed by Mcintosh in the same Nemertean, or by Keferstein in the Polyclad Turbellarian Leptoplana tremellaris.