HYDROZOA 549 with blood-lymph space distinct from digestive canal. 1 With the attainment of the coslomate condition, the two fundamental cell-layers, ectoderm and endoderm, which still appear in the embryo, become so far interwoven, and their products so highly differentiated, that it is no longer possible to recognize them as anatomical structures in the adult. The only deep-seated distinction between Hydrozoa and Anthozoa (the Actinozoa being thus termed when the Ctenophora are detached from them) appears to be the particular differentiation of the archenteric space in Anthozoa which has just been noted. It is no longer possible to separate the two groups from one another as Exoarii and Endoarii, as was proposed by Rapp (Ueber die Polypen im Allgemeinen und die Actinien insbesondere, Weimar, 1829) the first term indicating the Hydrozoa as possessed of external generative organs, whilst by the latter term the Anthozoa are pointed to as having internal generative organs. 2 This distinction breaks down completely in the case of Lucernaria, and even in that of the so-called phanero- carpous and some other medusae which discharge their genital products by the mouth, and quite rarely by rupture of the outer body-wall. The tendency to form calcareous deposits in the deep layers of the ectoderm, or mesoderm, as it has been termed, exhibited almost universally by the Anthozoa (whence the name Coralligena applied to them), is distinctive of them, though it has been shown first by Louis Agassiz, and more fully and recently by Moseley, to be paralleled among Hydrozoa, by the external calcareous deposits of the abundant and widely distributed Millepores and Stylasterids. A minute distinction between Hydrozoa and Anthozoa, which does not, however, hold good uni versally, is found in the form of the barbed threads ejected by the nematocysts. Instead of the complicated forms present in the latter group, the Hydrozoa are usually pro vided with either an unbarbed thread or one in which the barbs are confined to three at the base and a few minute barblets (tig. 5). Fundamental Forms of the Hydrozoa. The diblastula derived from the egg of a hydrozoon, when provided with a mouth, may be spoken of (as are the equivalent forms in other animals groups) as a person. Either this person elongates and develops tentacles in a circlet around or near the mouth, and usually becomes fixed by the aboral pole of the sac-like body, or the sac gradually assumes the form of a clapper-bell or of an umbrella with greatly thickened handle, the mouth being placed at the free end of the handle or of the clapper, and the animal freely swimming by the contractions and expansions of the dome of the bell -(disc of the umbrella). The two forms of persons are known, the former as the "hydriform" (2, 3 in fig. 16), the latter as the " medusiform " (4, 5, 6 in fig. 16). The HYDRIFORM PERSONS usually occur as fixed branching ^colonies or trees (figs. 36 and 37) produced by lateral budding from an original hydra-form developed from a diblastula. The hydriform person in its most fully developed state is seen in the colonies of Tubularia. In such a colony a number of hydriform persons are united like the flowers of a plant on its branches (whence Allman s terms hydranth, hydrophyton). Each hydriform person (fig. 35) has an elongated body with oral and aboral pole. The mouth is placed centrally at the oral pole, which is somewhat enlarged and conical. At the apex of the cone, immediately around the mouth, is a circlet of small tentacles ; at the base of the cone is a second circlet of larger tentacles; the surface of the oral cone is termed the hyposlome. In other genera 1 The Knterozoa or Metazoa admit of division into two grades (1) the Ccclentera, including sponges, polyps, jelly-fish, and corals, and .(2) the Ccelmiuzta, including all remaining forms. 1 See, however, note to the paragraph headed Definition of the Jlydrozoa, p. 555. (e.g., Hydra, fig. 42) the smaller circle of tentacles IB wanting ; in others, again, the tentacles are irregularly placed and not concentrated into one circlet (fig. 38). We regard the former as the typical condition. In the hydriform persons of the Scyphomedusce (figs. 26 and 27) the vertical axis is much shortened, the hypostome is flat, and the whole body cup-like or hemispherical. The tentacles of the hydriform person are sometimes hollow (Hydra, Garveia nutans, Hydrocorallince), being mere prolongations of the sac-like body ; but usually, though the endodermal cell-layer is continued into them, they are solid (2 in fig. 16). Very generally the tentacles of the hydra-form are indefinite in number, but in those belonging to the group of Scyphomedus<K a primary series indicating four radii (perradial) can be distinguished, to which are added four intermediate to these, marking four secondary radii (interradial), whilst eight more placed between the eight of the perradial and interradial series are known as adradial tentacles. The surface of the hydra- form may be entirely naked, or encased in a horny tube (perisarc) formed by the ectoderm : this may be confined to the aboral portion of the hydranth and to the common stem which unites the persons of a colony, or it may rise up and form a cup (or hydrotheca) around the oral region of the hydranth (figs. 32 and 33). The bodies of all hydriform persons, as well as the ten tacles, are excessively contractile, and when hydrothecse are present can be withdrawn into them. The ectoderm or outer cell-layer furnishes the protective and contractile tissues of the hydra-form. Very usually it is not more than one or two cells deep, and is sepa rated from the endoderm by a structureless lamella of firm consistence. In Hydra __ large cells of the ectoderm F ioT Epide (neuro-muscular cells of Kleinenberg) bound the external surface (fig. 3) and give off horizontal muscular processes which lie side by side on the structureless lamella forming thus a deep muscular coat, the fibrous elements of 7??, muscular - fibre processes. Kleinenberg, from Gegenbaur.) (Afttr F, . 4.-Portion of tlie body-w.,11 of Hi/tlra, showing ec oderm cells above, separated by "structureless lamella" from three flagellate endoderm cells below I he latter are vacuolated, and contain each a nucleus and several dark Ci-mulos In the middle ectoderm cell are seen a nucleus and three nemato- cvsts, with triirger hairs projecting beyond the cuticle. A large nernatocyst with everted thread, is seen in the right-hand ectudermal cell. (After F. t. Schulze.) which are not independent cells. In larger species some of the fibres may become separated from the tegumentary or superficial cells, and acquire the character of independent nucleated corpuscles (Hi/dractinia, Van Beneden). No nervous elements nor sense-organs occur in any hydra-form (except perhaps the Lucernariie). In Antennularia some ectoderm cells are amcebiform, and project processes which
change shape (uematophors). Tactile hairs (palpocils),
