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MESOZOA

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Originally appearing in Volume V18, Page 188 of the 1911 Encyclopedia Britannica.
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MESOZOA . See also:

Van Beneden i gave this name to a small See also:group of See also:minute and parasitic animals which he regarded as inter- mediate between the See also:Protozoa and the See also: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 See also:groups. Class I. RHOMBOZOA (E.vanBeneden).—These animals consist' of a central See also:cell from which certain reproductive cells arise, enclosed in a single layer of flattened and for the most See also:part ciliated cells; some of them are modified at the anterior end and See also: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 See also:Cambridge Natural See also:History, vol. ii. "See also:Worms, cap, but whose anterior &c.," by permission of See also:Macmillan & Co. Ltd. After ectodermal cells contain Gamble.) refringent bodies and A. Full-grown Rhombogen with in ocyema, v. See also:Ben. in See also: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. See also:

Nucleus of ectoderm cell. fixed in the renal cells of n. end. Nucleus of endoderm cell. their See also:host by their polar p." Calotte:" cap, the Heterocyemida B. Developing infusoriform embryo. are See also:free. The number of C. One fully See also:developed. ectoderm cells apart from D. " Calotte " of nine cells, the polar cap is few, some fourteen to twenty-two. i See also: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 See also:division of the smaller blastomere.

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

process. Each of the two small cells are now called ' See also:primary germ cells," and they enter into and See also:lie inside the large central cell. The primary germ cells See also:divide until there are eight of them all lying within the axial cell. At this See also:stage the future of the See also:parasite may take one of two directions. Following one path, the See also: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 See also: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 See also:swimming through the renal fluid attach themselves to the renal cells. They never leave their host, and See also:die in See also:sea-See also:water. The infusoriform larvae have a very complicated structure; they See also: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 See also: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 See also:

female. 2. Cylindrical female. which then give rise to males again, whereas the See also:females which produce the vermiform embryos arise from unfertilized vermiform larvae. After the infusoriform larvae have, See also:left the parent's See also: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 See also:affinities with the Rhombozoa. The genus, Haplozoon: of which two See also: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 See also:

mass of eggs destined to form male and female reproductive cells surrounded, by a single layer of ciliated ectoderm cells arranged in See also:regular rings which contain varying See also:numbers'"be., rows of cells. See also:Muscular fibrils occur between the See also:outer and inner cells. The sexes are See also:separate and unlike; and there are-tiro .kinds of females, cylindrical and See also: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 See also:appearance, and in See also: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 See also:ring is devoid of See also: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 See also: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 See also:

protoplasm; germ cells arise which develop into ciliated larvae and ultimately into males and females which only See also: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 See also:original See also:idea that in the Rhombozoa and Orthonectida we had animals intermediate between the Protozoa and Metazoa is no longer widely held. The See also:modern view is that the simplicity of their structure is secondary and not primary, and is correllated with their parasitic See also:habit of See also:life. They are probably derived from some Platyhelminthine ancestor and perhaps come nearer to the Trematoda than to any other group.

End of Article: MESOZOA

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