ORDER ACTINOIDEA. 57 



consists of siliceous spiculfe, and has founded upon this character 

 his genus Hyalonema.* 



51. Mr. B. Silliman, Jr., has been engaged in a series of analyses 

 for this work, the detailed results of which will be given in the 

 Appendix. We merely state here, that he has found in most of 

 the calcareous corals examined, a small per-centage of magnesia, 

 alumina, iron, silica, phosphoric acid, and fluorine, besides the car- 

 bonate of lime, which constitutes, after separating the animal matter, 

 from ninety-seven to ninety-nine per cent. The horny stem of the 

 Gorgonia setosa afforded him a considerable proportion of alumina, 

 besides phosphoric acid, some carbonate of lime, and ninety-three per 

 cent, of animal matter. 



We have a sufficient source for the elements of these ingredients 

 in the food of the polyps united with the waters of the ocean. 

 Through their animal functions, such changes and recompositions 

 take place, in the material thus received, as are recmired for the coral 

 secretions. 



III. Reproduction by Buds — The Compound Structure. 



52. In the preceding pages, on the Actinoidea, we have considered 

 merely the simple polyp, in which reproduction takes place only by 

 ovules, produced from the internal lamellae, and escaping by the 

 mouth of the parent-animal. These polyps, in very many species, 

 increase also by buds, and thus, from the single parent, perhaps but a 

 fraction of an inch in size, zoophytes may spread their branches to a 

 height of many feet. As we find the origin of the various forms of 

 corals, in this power of developing buds, connected with some pecu- 

 liarities in the animals themselves and their mode of growth, this 

 subject is one of prominent interest. We may first consider simply the 

 process of budding, and afterwards point out the different modes by 

 which the budding process gives rise to the forms of zoophytes. 



1. The Process of Budding. 53. Buds proceed from different 

 parts of a parent-polyp : those from the sides are called lateral or 

 inferior buds ; and those from the upper extremity, either just exte- 

 rior to the tentacles, or from the disk, are terminal or superior buds. 



The nature of the process is, in general, very similar to that de- 

 scribed under the Hydroidea (\\ 15, 16). In lateral budding, a small 

 protuberance appears on the side of the parent, into which the visce- 



* Proceedings of the Zoological Society of London, 1835, p. 62. 



15 



