. Text-book of zoology for junior students. Zoology. 84 INVEKTEBRATE ANIMALS. appear to be present, as a rule; but tlie mesoderm is usually pro- vitled with scattered nerve-cells. No definite organs of the circula- tion are develojied. Distinct reproductive organs are always present, and true sexual reproduction occurs in all the members of the class. In a great many forms, however, of the Actinozoa, we have com- jiosite organisms or colonics, produced by a process of "continuous" gemmation or fission, the zooids thus oi-iginated remaining attached to one another. In these cases—as i
. Text-book of zoology for junior students. Zoology. 84 INVEKTEBRATE ANIMALS. appear to be present, as a rule; but tlie mesoderm is usually pro- vitled with scattered nerve-cells. No definite organs of the circula- tion are develojied. Distinct reproductive organs are always present, and true sexual reproduction occurs in all the members of the class. In a great many forms, however, of the Actinozoa, we have com- jiosite organisms or colonics, produced by a process of "continuous" gemmation or fission, the zooids thus oi-iginated remaining attached to one another. In these cases—as in most of the Corals—the separate beings or zoiiids thus produced are termetl " polypes," the term " polyjjite " being restricted to the Hydrozoa. In the simple Aftiiiozoa, however, such as the Sea-anemones, the term " polype " is. Fig. 47. — Di;i-i'aiiiinatic section of a Si'a-Anernon3. a Month; .5 Stomach; & Borty-ca\-ity; c c Convolnted corfis (" crasperla") containiii^^ thread-cells, and forn^ing the fi-ee edges of tlie mesentery (jji); t f Tentacles; n Reprodnctive organ contained within the mesentery. The ectoderm (e) is indicated by the broad external line, the endi^derm (t') hy the thin line and the space hctwecn that and the ectudeim. a]iplied to tlic entire organism, as of no more than a single alimentary region. It follows fi'om this, that the entire Imdy, or " actinosoma," of any Actinozoun, m;iy be cimtp(_)sed either of a single polype, or of several such, produced by budding or cleavage, and imited to one anfither Viy a common connecting structure or cwnosarc. Most of the j\iiii-i(>\i«i ai-e permanently , like most Corals; some, like the , a limited amount of locomotive power; ;ind idle onler, Ihe <'ti-nnpliorii, is composed of highly active free-swiniming oiganisnis. Sunie of them are unjirovided with hard sli iictnres nr sujiports of any kind, as
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1885
