ECHINODERMS. 



137 



the alimentary tract. Bilateral symmetry is not so evident in most forms, though 

 it exists in all, and in the young is especially well marked. The line dividing the 

 body into two similar halves passes through the centres of the madreporic body and 

 of the central disc. 



The internal organs are far more complex than those of the coelenterates. The 

 most striking feature is that the digestive canal is entirely distinct from the body 

 cavity. In the higher forms this canal is tubular in form, and as it is much longer 

 than the body cavity, it is coiled in a spiral, which in all, except the serpent-stars, is 

 coiled from left to right. An anus is usually present. Usually there are connected 

 with the functional stomach a number of glandular pouches, which secrete a bitter 

 fluid, and appear to represent the liver of higher animals, Organs which act as teeth 

 are frequently present around the mouth. 



The so-called water vascular system is complex and peculiar, presenting several 

 interesting features. From the 

 under internal surface of the mad- 

 I'eporic body a canal goes down to 

 a circular tube surrounding the 

 mouth. From the fact that this 

 canal contains lime in its compo- 

 sition, it is known as the stone 

 canal. From the circular circum- 

 oral canal (ring canal) a branch 

 follows the centre of each ambu- 

 lacral area to its extremity. Con- 

 nected with these radial canals 

 are the ambulacra. These ambu- 

 lacra are arranged in pairs, and 

 consist of two portions ; an outer 

 part terminating in a sucking disc, 

 and an inner sac known as an am- 

 pulla. Between the two a tube 

 goes to the radial canal. Another 

 feature is sometimes present. Arising from the ring canal are from one to ten sacs 

 known from their discoverer, Poli, as the Polian vescicles. 



The physiology of this water vascular system is not clearly understood. All that 

 can certainly be said is that by the action of the muscular walls of the ampullsB and 

 the Polian vescicles water is forced into and withdrawn from the tubular ambulacra, 

 thus extending and retracting these organs. In the extremity of each ambulacrum 

 is a calcareous plate, to which are attached minute muscles, which by drawing in the 

 external integument form a vacuum similar to that which a boy forms with his wet 

 leather disc and a string. The strength with which these minute feet will cling is 

 remarkable, and the long stalk of the foot may frequently be torn in twain without 

 detaching the foot from the object to which it has become fastened. 



Connected with this water vascular system, which is locomotor and possibly also 

 respiratory, is another which is regarded as the true vascular or circulatory sys- 

 tem. It consists of two rings, one surrounding the mouth, just below the ring canal, 

 while the upper surrounds the anus at the opposite pole of the body. These two 

 rings give off branches, and are connected by a tube which follows the course of 



Fig. 126. — Diagram of water vascular system of a star-fish; a, mad- 

 reporic body; 6, stone canal; c, ring canal; d, radial canals; 

 e,f, ambulacra and ampullsc (a few only shown). 



