3 



polype depends on the colour of the food, and that those 

 granules which are nearest, the gastric surface are the first 

 to change and so in gradation from the stomach to the 

 external surface.* There appears to be no anatomical dif- 

 ference between the granules of one part of the body and 

 another ; for if the animal be turned inside out, the outside 

 will perform the function of digestion as perfectly as the 

 original gastric surface, and the young will frequently sprout 

 from the tentacula as well as from other parts of the body. 



The horny or sheathed Genera vary a great deal in form, 

 density, and the elaboration of their various parts, and have 

 a very graceful appearence. Their form is more or less 

 arborescent, and through their centres runs a granular pulp, 

 which terminates, at the extremities of the branches, in 

 polypes, which are modifications of the pulp and formed 

 from it. In the Hermia the sheath is nearly rudimentary, 

 and forms an imperfect cohering for the upper part of the 

 pulp; in the Tubularia the polypes always protrude beyond 

 the tubes, which cover the pulp ; in the Thoce, Sertularice, 

 Plumularice, Campanularioe, fyc, the polypes are furnished 

 with cells, into which they retire either from satiety, alarm, 

 or for rest. In some the cells are sessile, in others on ringed 

 footstalks ; they are cup shaped and arranged in one or two 

 rows on the stems or branches. At certain seasons of the 

 year, more especially about summer and autumn, but differ- 

 ing in different species, there are other larger cells formed, 

 which are the ovarian vesicles, which drop off as soon as the 

 ova or gemmules are perfected. 



The mode of reproduction varies, not only in the different 

 genera, but in the same under different circumstances. In 

 the naked Hydroida the young sprout from the sides of the 

 parent as perfectly formed animals, exercising the functions 

 of independent life, even before they become separated. 

 The young after remaining attached for some time, are 

 thrown off by a vital process and the cicatrix becomes ob- 

 literated ; so that no point can be observed on the old polype 

 to indicate the former situation of the young. This form 

 of reproduction sometimes takes place so rapidly, that the 

 young even to the third or fourth generation have young 

 before the first is separated from the parent ; this gives the 

 whole a very grotesquely branched appearance. 



In the Sertulariadce external ovarian vesicles are formed, 

 which contain the reproductive gemmules. These are 



* Trembly, Hist, des polypes p. 132. Roget's Bridgwater Treatise, 

 vol. 2, p. 77 and 78. Mag. of Zool. and Bot., vol. l,p. 235. Rote *. 



