334 Descriptive Zoology. 



passes down through a madreporic canal or stone canal 

 (made of the same material as the skeleton), and this 

 stone canal empties into the circular water tube around 

 the mouth. Thus water, filtered through the madreporic 

 plate, is supplied to the whole water tube system. There 

 is also a series of water bulbs opening into the circular 

 water ring ; these are the Polian vesicles. 



How the Starfish Crawls. When the starfish wishes to 

 crawl, it distends the tube feet by the action of the water 

 bulbs (or water sacs, called ampulla) along the inside of 

 the rays. The flattened disklike ends of the feet are 

 closely applied to the surface on which it is crawling. Then 

 the center of the disk is somewhat retracted, making a sort 

 of " sucker," by which the foot holds firmly. When many 

 feet are thus holding there is considerable power, and when 

 the feet are now shortened the body as a whole is pulled 

 along. The starfish can climb vertical walls or even cling 

 to the under side of a horizontal surface. And so strongly 

 can it hold that sometimes, when the collector tries to pull 

 it away he simply tears the starfish in two. The starfish 

 can crawl with any ray foremost. 



The Digestive System of the Starfish. The mouth opens 

 into a large stomach which fills nearly all the space in the 

 central body, and has, in addition, a large lobe extending a 

 short distance into each ray. The stomach is thin-walled 

 and very extensible. This wide part of the stomach is 

 called the cardiac portion. The stomach narrows above, 

 and again widens to form the pyloric portion. Into this 

 portion is poured the secretion of ten digestive glands, a 

 pair in each ray. The ducts of the two glands in each 

 ray unite, so there are five ducts entering the pyloric 

 stomach. These glands are like long bunches of small 

 grapes. The glands are held in place by thin folds of the 



