236 CCELENTERATA. 



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, Meduscu 

 (Aeginopsis, Pelayia] 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 (8cy2Jhomedusce), 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 buds 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. HYDROJIEDUS/E.* 



Colonial forms, the individual Polyps of which are without oesophageal 

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

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

 Jfedusce) 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 are frequently surrounded 

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

 structures may become extended into cup-like hydrothecse 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 fiuid. 



The Polyps have no eesophageal tube, and the ciliated gastric 

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

 entoderrn 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.. 1860 1862. G. J. Allman. -A Monograph of the 

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

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

 system uncl die Sinnesorgane der Medusen,' 1 Leipzig, 1878. 



