MESOZOA  .

Van Beneden i gave this name to a small group of minute and parasitic animals which he regarded as inter- mediate between the Protozoa and the Meta- zoa . The Mesozoa com- prise two classes: (i) the Rhombozoa, which are found only in the kidneys of Cephalopods, and(2)the Orthonectida, which infest specimens of Ophiurids, Poly- chaets,Nernertines,Tur- bellaria and possibly other groups . Class I . RHOMBOZOA (E.vanBeneden).—These animals consist' of a central cell from which certain reproductive cells arise, enclosed in a single layer of flattened and for the most part ciliated cells; some of them are modified at the anterior end and form the polar cap . The Rhombozoa comprises two orders : (a) Dicyemida, ciliated vermiform creatures whose polar cap has 8 or 9 cells arranged in two rows (Dicyema, Koll., Dicyemennea, Whitm.) ; (b)Heterocyemida, non-ciliated animals with no polar (From Cambridge Natural History, vol. ii . "Worms, cap, but whose anterior &c.," by permission of Macmillan & Co . Ltd . After ectodermal cells contain Gamble.) refringent bodies and A . Full-grown Rhombogen with in ocyema, v . Ben. in Octopus fusoriform embryos (emb). vulg¢ris; Microcyema in g . Part of endoderm cell where forma- Sepiao8icin¢lis) .

Unlike tion of the embryos is actively proceeding. the Dicyemida, which are n. ect .

Nucleus of ectoderm cell. fixed in the renal cells of n. end . Nucleus of endoderm cell. their host by their polar p." Calotte:" cap, the Heterocyemida B . Developing infusoriform embryo. are free . The number of C . One fully developed. ectoderm cells apart from D . " Calotte " of nine cells, the polar cap is few, some fourteen to twenty-two . i Bull . Ac . Belgique (1876), p . 35 . The central cell is formed by the layer of the first twoblastomeres, and remains quiescent until surrounded by the micromeres or products of division of the smaller blastomere .

It then divides unequally, and of the two cells thus formed the larger repeats the

process . Each of the two small cells are now called ' primary germ cells," and they enter into and lie inside the large central cell . The primary germ cells divide until there are eight of them all lying within the axial cell . At this stage the future of the parasite may take one of two directions . Following one path, the animal (now called a " Nematogen ") gives rise by the segmentation of its primary germ cells to vermiform larvae which, though smaller, are but replicas of the parent form . Following the other path, the animal (now termed a " Rhombogen ") gives origin to a number of " infusoriform larvae," several of these arising from each primary germ-cell . The vermiform larvae leave their Nematogen parent and swimming through the renal fluid attach themselves to the renal cells . They never leave their host, and die in sea-water . The infusoriform larvae have a very complicated structure; they escape from the Rhombogen, and, unlike the vermiform larvae they can live in sea-water . They possibly serve to infect new hosts . Some authorities look upon these infusoriform larvae as males, and consider that they fertilize some of the Nematogens, (From Cambridge Natural History vol. ii., "Worms, &c.," by permission of Macmillan & Co . Ltd .

Alter Julio.) Full grown male . T . ' Flattened form of

female . 2 . Cylindrical female . which then give rise to males again, whereas the females which produce the vermiform embryos arise from unfertilized vermiform larvae . After the infusoriform larvae have, left the parent's body, the, Rhombogen takes to producing vermiform offspring, and thus becomes a secondary Nematogen . Thus, if the above views be correct, a Rhombogen is aprotandrous hermaphrodite . E . Nerescheimer has recently described under the name of 'Lohmanella catenata an organism parasitic in Fritillaria which shows marked affinities with the Rhombozoa . The genus, Haplozoon: of which two species have been found in the worms "Travisia and Clymene byDogiel is classed as a new group of Mesozoa . Class II .

ORTHONECTIDA (A . Giard).`---The Orthonectida contain animals with a central

mass of eggs destined to form male and female reproductive cells surrounded, by a single layer of ciliated ectoderm cells arranged in regular rings which contain varying numbers'"be., rows of cells . Muscular fibrils occur between the outer and inner cells . The sexes are separate and unlike; and there are-tiro .kinds of females, cylindrical and flat . There are but two genera, Rhopalura and Staecharthrum, the latter found in a Polychaet . The male R. giardii lives in the body-cavity of Amphiura squamata, has six rings of ectodermal cells all ciliated except the second, 'whose cells contain 'refringent granules . The ectoderm encloses the testis, a mass of cells which have arisen from a single axial cell in the embryo . The female differs from the male in appearance, and in size it is larger . It occurs in two forms: (I) The cylindrical with 8 (or q) rows of ectoderm cells; here as in' the male the second ring is devoid of cilia: (2) The flat females are broader, 'uniformly.tiliated; and have rrot rings of ectoderm cells . The central mass of cells forms ova which are free in the cylindrical forms; they leave the mother through the dehiscing of the cells of the non-ciliated ring, are fertilized and develop parthenogenetically into females both flat and cylindrical . R. pelseneeri and S. giardi are said to be hermaphrodite . The parasites first make their appearance in a host in the form of a plasmodium comparable with the sporocyst of a Trematode .

By the segregation of nuclei and some of the surrounding

protoplasm; germ cells arise which develop into ciliated larvae and ultimately into males and females which only discharge their spermatozoa and ova when they reach sea-water . The product of the consequent fertilization is unknown; presumably it infects new hosts; entering them in the form of a nucleated plasmodium . The original idea that in the Rhombozoa and Orthonectida we had animals intermediate between the Protozoa and Metazoa is no longer widely held . The modern view is that the simplicity of their structure is secondary and not primary, and is correllated with their parasitic habit of life . They are probably derived from some Platyhelminthine ancestor and perhaps come nearer to the Trematoda than to any other group .