3 



polype depends on the colour of tlie food, an<] tliat those 

 grannies which are nearest the gastric surface are the firnt 

 to change and so in gradation from the stoiDach 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 he turned inside out, the oHtside 

 will perform the function of digestion as perfectly as the 

 original gastric surface, and the young will frequenily sprout 

 from the Icntacula as well as from other parts of tlie body. 



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

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

 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 Hcrmia the sheath is neatly rudimentary, 

 and forms an imperfect covering for the upper part of the 

 pulp; in the Tnbularia the polypes always protrude beyond 

 the tubes, which cover the pulp ; in the Tho(e, Sertitlarice, 

 Plumularice, Campanularioe, 6fC. 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 sliaped and arranged in one or two 

 rovvs on the stems or branches. At certain seasoBs of the 

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

 ing ill 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, hot in the same under (iifferent circumstances. la 

 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 pol}j>e 

 to indicate the fortner situation of the young. This loim 

 of reproduction someliuies takes place so rapidly, that ihe 

 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 SertulariadcB exiernal ovarian vesicles are formed, 

 which coDtain the reproductive gemmules. These are 



* Trembly, Hist, des polypes p. 132. Roget's Brulgwater I'leatisr, 

 vol. 2, p. 77 and 78. Mag. of Zool. and Bot., vol. 1, p. Jjj, >.oi(i •, 



