260 



THE POPULAR SCIENCE MONTHLY. 



the branches are minute cells (indicated by the fine dots in the wood- 

 cut), each of which was the seat of one of the little hydra-like animals 

 (in this case not more than the fourth of a line long), and usually with 

 short tentacles spread out star-like. 



" We will now pass to the true polypes. These may be divided 

 into those which secrete coral and those which do not. The latter 



Fig. 3. 



Fig. 4. 



A n Horizontal Section of a Polype showing 

 the Internal Arrangement of tiie Folds and 

 Compartments. 



Coral from the West Indies, showing the Structure 

 of the Cells. 



have soft, leathery bodies, and live attached to stones and other 

 substances upon the sea-bottom, by a basal, sucker-like disk. They 

 have the power of locomotion by contraction and expansion of the 

 muscles of the disk. But the coral-making polypes are fixed to 

 the stone which they create, and which is part of themselves. The 

 polype is the living part of the coral, the gelatinous mass which 



Fig. 5. 



Multiplication of Polypes by Spontaneous Fission. 



It consists of a sac 

 opening from the 



fills the radiating cells upon the coralline surface. 



or stomach, and an enveloping membrane. An 



stomach outward is the animal's mouth. This is surrounded by 



tentacles, which by their motion aid in bringing to it currents of water 



in which floats its food, and of the solid matters of which it constructs 



its calcareous skeleton." In the polype, the stomach or digestive sac, 



with its appendages, constitutes the whole animal. Into the stomach 



