PHYLUM ECHINODERMA 171 



the external openings of the gonoducts. The five pairs of gonads 

 He within the eoelom at the bases of the arms. One genital plate 

 is in most instances enlarged and perforated by numerous pores 

 which serve for the entrance of water into the water-vascular 

 system. In some instances, this sieve or madreporite is not 

 single but occurs as two or more plates. 



S3niimetry. — In the typical condition of a single madreporite, 

 the eccentric position of this organ is the most conspicuous 

 external evidence of deviation from the radial type of symmetry. 

 Since a plane passing through the madreporite and through the 

 arm on the opposite side of the disc bisects the body, the animal 

 is in reality bilaterally symmetrical. For convenience of refer- 

 ence, the arm opposite the madreporite is called the anterior arm 

 or ray. This, and the two adjacent rays, constitute the tri- 

 vium, while the two remaining rays, between which the madre- 

 porite is located, comprise the bivium. 



The digestive system opens to the exterior through the ventral 

 mouth. Small objects are ingested through the mouth; but 

 because of the small size of the peristome, large objects cannot 

 be taken into the body. Mussels and oysters (Fig. 83), which 

 serve as food for the starfish, are digested outside the body 

 through the peculiar provision which admits of the starfish 

 everting the stomach through the mouth opening. The everted 

 stomach surrounds large food masses and, after digesting them, 

 is again drawn through the mouth opening into the body. A 

 rather conspicuous constriction divides the stomach into dorsal 

 and ventral chambers. The mouth opens directly into the 

 ventral or cardiac chamber of the stomach while dorsal to this 

 lies the pyloric chamber. From the cardiac chamber, a gastric 

 pouch extends into each ray. A pair of hepatic ceca, occupying 

 much of the space within each ray, communicate by a common 

 duct with the pyloric chamber near the base of each arm. An 

 intestine of minute size leads from the pyloric chamber to the 

 aboral surface of the disc where it either opens through an eccen- 

 tric anus or ends blindly. Small, branched ceca are given off 

 from the intestine in some starfishes. The fact that these 

 ceca undergo rhythmic pulsations suggests the possibility that 

 they may have a function as respiratory organs. 



The water -vascular system is a series of tubes or canals of 

 which the main parts comprise a ring canal surrounding the 

 esophagus and a series of radial vessels given off from this, one 



