clvi Introduction. 



elsewhere in the sub-kingdom, evidence is only obscurely borne in the 

 mode of growth of the Echinoidea, and possibly also in the spontaneous 

 self-mutilation and division observable amongst the Holothurioidea. 



See, for the development of Antedon Romceiis, [Comafula Rosacea,) 

 Professor Wy villa Tliomson^ Phil. Trans., 1865 ; Dr. Carpenter, 

 ibid., 1866. 



Sub-kingdom, Coelenterata. 



Radially-arrang-ed or bilaterally-symmetrical animals, in which 

 the general cavity of the body and that of the dig-estive sac are 

 always continuous. They may be fixed or free, solitary or social ; 

 they are exclusively aquatic, and almost exclusively marine. The 

 walls of the body consist always of two layers, an ectoderm and an 

 endoderm ; both, but especially the outer layer may become in- 

 durated by interstitial deposit; and the outer layer may secrete a 

 more or less hard exterior cell. Both layers, but especially the 

 exterior, are provided with ' thread- eells,^ and both layers, but es- 

 pecially the interior, are, at one period or other, ciliated. The mouth 

 is the only outward opening- of the digestive tract ; it is ordinarily 

 surrounded by a corona of tentacles, the hollow interior of which 

 is continuous with the general body cavity. This cavity may be 

 a simple and direct prolongation of the digestive sac ; or this sac 

 may, whilst suspended by its sides in the perigastric space, open at 

 its bottom into it. The interior of the stomach and of the general 

 body cavity being ordinarily ciliated, circulation is maintained in 

 the alimentary fluids; but there is no tubular vascular system of 

 any kind, nor any specialized respiratory apparatus in these animals. 



The Sub-kingdom is divisible into three Classes, the Ctenophorae 

 of which the Cesium Veneris., the Cydippe, and the Beroe may be 

 taken as examples ; the Anthozoa ; and the Hydrozoa. 



A nerve-system has been supposed to exist centrally in the Cte- 

 nophorae ; and peripherally, as special sense organs in the 3Iedusae 

 in their marginal cysticles, but much doubt has been justifiably 

 raised as to the really nervous character of the structures in 

 question. (See Claus, Zeitschriffc Wiss. Zool., xiv., 1864, pp. 385, 

 388). The Ctenophorae are hermaphrodite ; the Anthozoa and Hy- 



