STRUCTURE OF CORALS. 113 



impregnated, until it is hatched, and the progeny, a pear-shaped 

 ciliated embryon, is discharged into the water by the mouth. Why- 

 is it not dissolved with the food? There is one other part of the 

 organism of these creatures which I must now notice briefly, viz., 

 their thread cells, which are endowed with the most marvellous 

 stinging powers, and from which the Acalephse take their name. 

 With the exception of some of these, the thread cells of the sea 

 anemone are more highly developed than in other animals of the 

 class. During the act of contraction on being touched, a number of 

 long filaments called Craspeda (fringes) issue through the walls of 

 the body by almost imperceptible slits, with a stream of water 

 from the chambers. These are covered with the poisonous stinging 

 cells, by which the creature touched or entangled in them is para- 

 lysed and rendered an easy prey. I call your special attention to 

 these as interesting objects for microscopical examination and 

 display. It is easy for you now to understand how the stony 

 Corals and Madrepores are formed. The Coral polypes have the 

 power of abstracting carbonate of lime from the sea-water, com- 

 bined with a little animal matter, and a still smaller quantity of 

 phosphate and sulphate of lime, with a trace of silver and magnesia. 

 This stony substance takes the form of needles, a network is 

 formed around the body of the animal, which, by a series of these 

 deposits, is condensed into a hard impervious coat. The same 

 process goes on simultaneously in the mesenteric plates near their 

 bases, and in many of the Corals a horny column forms in the axis 

 of the Polype, called the Columella, which is soon hardened by cal- 

 careous deposit, and varies in structure in different genera. The 

 horizontal septa which are to be seen more or less in all Corals, 

 crossing the mesenteric plates — some of them at varying depths in 

 each of the loculi, and called tabulas and dissepimenta — are evidences 

 of the growth of the tenant upwards, since having left that part it 

 has no longer any use for the space, and therefore cuts it off by a 

 partition. Thus, the Actinious polypes may be said to possess an 

 internal skeleton, and they also acquire an external one, in the form 

 of a cylindrical coat, into which most of them can withdraw the 

 soft upper part of their bodies and tentacles. The plates are some- 

 times continued through the wall, forming ribs, which project, but 

 are still covered by the ectoderm. By far the greatest number of 

 Corals grow in size by budding, as the Astrasa, one of the deepest. 

 They form groups in which the whole of the polypes, except their 



