23G CCELESTEBA.TA. 



widely distributed, especially amongst the polypoid forms, in which 

 it leads to the formation of polymorphous animal stocks. The two 

 forms of reproduction alternate for the most part in regular order, 

 i-o as to produce different generations. There are, however, Medusce 

 (Aeyinopsis, Petayia) which proceed without alternation of genera- 

 tions and develop directly from the ovum by continuous development 

 with metamorphosis ; but, as a general rule, the egg of the Medusa 

 (phanero-codonic gonophore) or the medusoid generative bud (adelo- 

 codonic gonophore) produces a Polyp, and this Polyp either at once, 

 by transverse fission (Scyphomedusce), or later, after a longer period 

 of growth, in which a sessile or free-swimming polyp stock is pro- 

 duced, gives rise to a generation of free-swimming Medusa?, or of 

 medusoid bnds which never become separate from the polyp stock. 

 The Hydromeduste feed entirely on animal substances, and for the 

 most part are inhabitants of the warmer seas. The free-moving 

 Medusce and Siphonophora are phosphorescent. 



Order 1 . HYDROMEDUS.E.* 



Colonial forms, the individual Polyps of which are without (ssopkageal 

 tube or inesenteric folds. The sexual generation has the form either 

 of small free-swiwminy Medusae provided with a velum (Craspedote 

 Medusce) or of medusoid yenerative buds (rudimentary Medusce) 

 which remain attached to the hydroid colony. 



The Polyps and polypoid forms are the asexual individuals. They 

 form small moss- or tree-like stocks which are frequently surrounded 

 by chitiuous or horny tubes (cuticular skeleton). These exoskeletal 

 structures may become extended into cup-like hydrothecfe surrounding 

 the individual Polyps. The stem and ramified branches [ccenosark] 

 contain a central canal which communicates with the gastric space of 

 each individual Polyp and polypoid appendage and contains the 

 common nourishing fluid. 



The Polyps have no ccsophageal tube, and the ciliated gastric 

 cavity is undivided by mesenteries. As a rule, the ectoderm and 

 entoderm remain simple, and are only separated by a thin interposed 

 supporting lamella which does not contain cells. The presence of 

 elongated muscle fibres as processes of the ectodermal epithelial cells 

 is very general (Hydra, Podocoryne). These muscles may, however, 



* L. A^assiz, "Contributions to the Natural History of tli-- Timed States of 

 America/' vol. ii. iv., 18(501802. G. J. Allman, "A Jlonuirmph of the 

 Gymnoblastic or Tubulariau Hydroids," vol. i. and ii., London, 1871 and 1872. 

 N. Kleinenberg, "Hydra." Leipzig, 1S72. O. and 11. Hertwig, "Das Nerven- 

 sys'etn uud die Sinnesorgane der Medusen," Leipzig, 1S7S. 



