20 MECHANICAL FUNCTIONS. 



notes a mass of these animals, and polype a single 

 animal. 



This order embraces many species of very simple 

 animals, chiefly inhabiting salt water. Some of them 

 are exceedingly minute, while others are several inches 

 in length. 



Each of these curious animals is formed of a tube, 

 attached by its lower end to some solid substance, the 

 upper end being surrounded by a number of flexible 

 fibres or arms, called tentacula. These tentacula radi- 

 ate from a common centre, in the midst of which is the 

 mouth of the animal. A single polype is represented 

 Fig. 3. by fig. 3. The tentacula are eight in number, 

 but in some species are much more numerous. 

 The arrangement of these, on the margin of 

 the mouths of the animals, bears a considera- 

 ble resemblance to a flower with radiating 

 petals, as the daisy and aster. 



Polypi for the most part reside in cells, or 

 tubes, composed of horny or calcarious matter, in the 

 form of sheaths, which enclose the body of the animal, 

 leaving the tentacula and mouths free for action above 

 their margins. Sometimes these tubes are joined to- 

 Fi-. 4. gether endwise, like the branches of a 



tree, leaving lateral apertures for the 

 protrusion of the tentacula of each 

 animal, as shown by fig. 4. In this 

 figure each bundle of radiating fibrils 

 along the branches represents the ten- 

 tacula of a polype. 



The well known marine substance 

 called coral is the product of the la- 

 bors of certain species of these indus- 

 trious animals. A great number of 

 species are perpetually employed in 

 the construction of different varieties of this substance. 

 Coral is composed of calcarious particles, with a portion 

 of animal, and occasionally coloring matter elaborated 

 into a solid, or porous form, by these animals. 



What does polype mean 1 Polypus ? Polypi 7 Describe a polype I 

 What are tentacula 7 What common flowers do these animals resem- 

 ble ? What does fig. 4 represent 1 



