262 PRACTICAL LESSONS IN SCIENCE. 



tive bud of the medusae produces a polyp. The radiating parts 

 are on the plan of four. 



The Echinodermata are popularly known as sea-urchins, star- 

 fish, feather-stars, etc. They are radiate animals on the plan of 

 five, and have a rough, often prickly skin, which has the power of 

 secreting calcareous matter. They have a distinct digestive canal 

 with mouth and vent, and a water vascular system, and a true 

 circulatory system are sometimes present. The nervous system 

 consists of nerves running down the five rays from a ring sur- 

 rounding the gullet. Sexual reproduction is the rule. These ani- 

 mals are inhabitants of the sea; are generally capable of a slow 

 creeping movement by means of suctorial tubes, feeding mainly 

 on mollusca and seaweeds. Many of them possess great repro- 

 ductive power, being able to replace lost parts with all their ap- 

 paratus of tubes, nerves, etc. 



The Crinoids, during the whole or a portion of their existence, 

 are fixed by a jointed flexible stalk largely composed of calcare- 

 ous matter. The radiating feather-like arms give the organism 

 something of the appearance of a flower, hence the name, meaning 

 lily -form. The crinoids have been very abundant since early geo- 

 logical times, their stems and crowns contributing largely toward 

 building up great beds of limestone. 



The Star-fish are characterized by their obviously star-shaped 

 body, the stomach, reproductive organs and the nervous system 

 all having the radiate arrangement. The color is yellow, orange 

 or red, the upper surface leathery, roughened by calcareous plates, 

 tubercles and spines. The mouth is below and the rays are fur- 

 rowed underneath, having numerous holes through which pass 

 the sucker-like tentacles or feet which serve as organs of locomo- 

 tion and prehension. They are voracious, and the most destruc- 

 tive enemies of the oyster. 



The Brittle-stars are similar to the star-fish, but the long arms 

 are mere appendages of the body and do not contain any portion 

 of the digestive canal. 



The Sea-urchin varies in form from a sphere to a disk. The 

 shell is composed of numerous calcareous plates, the whole cov- 



