236 CCELENTEEATA. 



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, 

 so as to produce different generations. There are, however, Medusce 

 (Aeginopsis, Pelagia) 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 niedusoid generative bud (adelo- 

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

 by transverse fission (Scyphomedusoa), 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 Medusae, or of 

 medusoid buds which never become separate from the polyp stock. 

 The Hydroinedusse. 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. HYDEOMEDUS^E.* 



Colonial forms, the individual Polyps of which are without wsophageal 

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

 of small free-swimming Medusce provided with a velum (Craspedote 

 Meduscey or of medusoid generative 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 aro frequently surrounded 

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

 structures may become extended into cup-like hydrothecte surrounding 

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

 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 oesophageal 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. Agassiz, " Contributions to the Natural History of the United States of 

 America/' vol. ii. iv., 186018*52. G. J. Allman, "A Monograph of the 

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

 N. Kleinenberg, " Hydra," Leipzig, 1872. 0. and R. Hertwig, "Das Nerven- 

 system und die Sinnesorgane der Medusen," Leipzig, 1878. 



