52 ANEMONES AND CORALS 



sides; The tentacles are hollow, and open into the interseptal 

 spac es, so that the fluid of the stomach can pass around and up 

 into all the soft internal parts. The mouth is extensible, has 

 muscular sides, and passes at once by a narrow space to the 

 digestive cavity below. Nematocysts (thread-cells) and cilia are 

 present in these parts as well as on the tentacles. 



Minute organisms floating in the water come in contact with 

 the top of the Coral, and are stopped by a mucus secretion or 

 by the nematocysts, and moved towards the mouth by the ten- 

 tacles, or by the motion of the cilia. The prey so captured dis- 

 appears inside the mouth to be digested, and the juices are cir- 

 culated from cell to cell, adding to the bulk of the polyp. The 

 calcareous parts of the prey, supposing it possessed a shell, and a 

 certain amount of the salts of lime held in solution by the sea- 

 water, are retained in the structures of the mesoderm of the body, 

 and out of this material the hard part of the Coral is formed, 

 the c arbonate of lime being deposited in long or short, slender 

 nee dies, or prisms, in the interstices of a peculiar connective tissue 

 of the mesoderm. 



A piece of Coral, when cut into thin slices or sections for exa- 

 mination under the microscope, shows numerous radiating lines 

 once occupied by organic matter, and starting from them on all 

 sides are masses of the prisms and needles of carbonate of lime. 

 This texture varies in different Corals, in some being very dense, 

 in others very lax and porous ; while in the latter type the tex- 

 ture of the hard part is very spicular, the ends being joined to 

 form a kind of cellular structure. It is upon these two distinctive 

 types of structure that the two great divisions of the Madreporaria 

 the Aporosa and the Porosa are based. 



The hard parts of the Coral are wonderfully regular in their 

 radiation and numerical structure. The theca, or wall, which forms 

 the cup of the Coral, is closed below at the base and open at the 

 opposite end, the calice. The thin vertical plates, or septa, 

 pass from the inside of the cup towards the central axis, and are 

 free above at the calice, and sometimes not joined to anything 

 in the centre of the cup. But there may be a columella in the 

 centre, which starts from the bottom of the cup and grows up- 

 wards ; or it may be formed by the ends of the septa. In some 

 Corals the interseptal spaces are open from top to bottom, while in 



