Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/564

548 548 HYDROZOA SaVClass II. Hydromedusae. Order 1. GriisoBLASTEA-AifT Fam. 1. Tubularidaa ( Tubularia (fig. 35). Hyboeodon. ( Corymorpha (fig. 34). ( BouKainvillia (fi 3. Eudendrida; .................. ! Perigonium. ( Lizzia (fig. 44). . llia (figs. 36, 37). 4. Cladonemida; 5. Bimendae ( Cladonema. j ciavatclla. j Garveia. j stvlactis. 6. Dicorymdae .................. Dicovyne (fig. 40). t SarsiadK (fig. 45). 7. Corynidaj .................... &amp;lt; Coryne. ( Syncoryne (figs. 41, 46). Order 2. CALYPTOBLASTKA-LEPTOMEDUSJB. Fam-LPlumularid* 2. Sertularida? . 3. Campanularidss j Sertularia. ( Halecium. f Eucopidse. J Cainpanularia (fig. 43). j Laomedea. ( Obelia. Thaumantias. 4. Thauma &amp;lt;Thaumantia ) Lafoea. ntiadC ^ Mclicertum. (Tima. I jEquorea. 5. ^Equorida3 ..................... Zygodactyla. (. lUiegmatodes. Ordsr 3. TRAcnojiEDtrs.s; (Haeckcl). Fam. 1. Petasids ..................... Petasus. 2. Trachynemidse ............... Khopalonema. 3. Aglauridse ..................... Aglaura. f Liriope. 4. Geryonlda ..................... Carmarina (figs. 48, 49). Order 4. XARCOMEDUS^E (Hacckel). Fam. 1. Cunanthida; .................. Cunina (figs. 50, 51). ,, 2. Pcganthidas .................. Polyxenia. 4.Solmand&amp;gt;B ..................... I Sohnissus { Solmaris. Order 5. HTDROCORALLINVE (Moscley). Fam. 1. Milleporidse ................... Millepora (figs. 52, 53). ( Sporadopora. 2. Stylasteridse .................. Distichopora. ( Astylus (fig. 54). Order 6. SIPHONOPHOBA. Sub-Order 1. Physophorida?. Fam. 1. Athorybiadte ................. Athorybia. 2. Physophoridse ............... Physophora (fig. 57, C). ( Forskallia. 3. Agalmidas ..................... 4 Halistemma. (.Agalma (fig. 57, E). 4. Apolemiadse .................. Apolemia. 5. Rhizopliysidaj ............... Hhizophysa. Sub-Order 2. Physalidaa. Fam 1. Physalidse ..................... Physalia. Sub-Order 3. Calycophoridae. Fam. 1. Hippopodiidaj ................ Gleba. ( Praya. 2.Diphyidaj ..................... 1 Diphyes (fig. 57. A). ( Abyla. 3. Monophyidaa .................. Sphasronectes. Sub-Order 4. Discoideas. f Velella. Fam. 1. Velellidse ( Porpita. The Ilijdrozoa present a greater simplicity of ultimate structure than do any animal organisms possessed of as great a complexity of external form. As in all Metazoa or Enterozoa, the life cycle of a hydrozoon starts with an egg which is at first a single cell or unit of protoplasm, but proceeds after fertilization to multiply by transverse fission in such a way that the resulting cells or units are arranged in two layers, each one cell deep, disposed around a central cavity the enteron or archenteron. The sac thus formed is known as a diblastula (figs. 1, 2, and 25). By the forma tion 1 of a mouth to the sac, the enteron acquires the functions of a digestive retort in which food matters taken in at the mouth are brought into a chemical condition suitable for the nutrition of the surrounding cells. The two layers of cells (of which the outer only acquires additional layers 2 1 In Ilydromediisce the inner layer of cells forms by delamination, in Scyphomedusce by invagination. In the latter case the sac closes up, and the mouth is formed by a new opening. 2 . It is probable that the numerous rows of cells described in the endoderm of Tubularia and Corymorpha by Allman, in his great mono graph of the Tubularian Hydroids, are due to a plication of the by the division of the primary cells, and that by no- means in all cases) received from Allman (Phil. Trans. f 1855) the names respectively of the ectoderm and the endoderm, having previously been shown by Huxley (1849) to be the fundamental mem branous constituents of which the most varied parts of the more com plex Hydrozoa such as tentacles, swimming bells, and air-bladders are built up in the adult condition. Huxley also pointed out the iden tity of these membranes with the two primary layers of the vertebrate embryo. The endoderm and the ectoderm, which present themselves, as is now known, in the diblastula (or gastrula) phase of all Enterozoa, re main in Hydrozoa (and also in the allied groups of Ccelentero) as permanently distinguishable ele ments of structure. This important disposition is associ ated with and dependent on the simple character which the archenteron or primitive digestive space retains. Into what ever lobes or processes the sac-like body may be, so to FIG. 1. Diagram of a Di blastula. a, orifice of in vagination (blastopore) 6, archenteric cavity ; c, endoderm : d, ectoderm. (From Gegenbaur s Ele ments of Comparative- Anatomy ) FIG. 2. Formation of the Diblastula of Eucope (one of the Calyptoblastic Hydro- medusce) by delamination. (From Balfour, after Kowalewsky.) A, B, C, three successive stages, ep, ectoderm; fiy, endoderm; al, enteric cavity. speak, moulded, whether tentacles 3 or broader expansions, into these the cavity of the archenteron is extended in the first instance ; and where the actual cavity is obliterated the endodermic cell-layer remains to represent it (Gefass- platte or endoderm-lamella, see figs. 7 and 16). Conversely, whatever canals or spaces are discovered in the substance of a hydrozoon (excepting only the cavity of ectodermal otocysts) are simple and direct continuations of the one original enteric cavity of the diblastula, and all such spaces are permanently in free communication with one another. 4 The whole of the Hydrozoa seem to present a lower grade of structure than the Actiitozoa, in so far as the latter, whilst retaining permanently free communication between all parts of the archenteric space, yet exhibit a differentia tion of this space into an axial and a periaxial portion a digestive tube and a body cavity. The differentiation has only to proceed a step further, namely, to the closure or shutting off of the axial from the periaxial portion of the archenteric space, and we obtain the condition which characterizes the adult forms of the Coelomata, or animals original endodermal cell-layer. The two kinds of cells in two layers figured by the same authority in the endoderm of Gemmcllaria implexa, pi. vii. fig. 5, cannot, however, be thus explained. 3 Some solid tentacles, with a single axial row of endodermal cells, form an exception to this statement. 4 The observations of Eilhard Schulze cited in the article CcELENTEKA do not form any real exception to this statement.