SHALLOW- WATER STARFISHES 21 



The mouth is central, dilatable, and surrounded by soft membrane. 

 The so-called jaws are merely the adoral ambulacral and adambu- 

 lacral plates, more or less modified and coalesced ; the " teeth " are 

 only slightly modified adambulacral spines, in this work called peroral 

 spines. (See text-fig. 3.) 



The stomach is very saccular and usually evertible. It usually has 

 a lobe and a pair of digestive glands extending into the cavity of each 

 ray; but in the case of some multiple-rayed species (Heliaster}, 

 it has lobes corresponding only to the primary five rays. In this 

 case the five stomach-lobes do not enter the rays, but the pairs of 

 digestive glands do. In some slender-rayed genera, also, the stomach 

 is confined to the disk. The intestine is usually nearly or quite 

 abortive and not functionally active. The so-called " anus " is a 

 dorsal pore, chiefly for the discharge of secretions from the dorsal 

 glands, or " caecal appendages," probably nephridial in function, 

 and called nephridial glands in this work. 



Commonly there is a single pair of branched gonads in the proxi- 

 mal part of each ray, with simple ducts discharging through a pair 

 of interradial pores, which may be either ventral or dorsal. In cer- 

 tain Brisingidae, Luidiidae, and in a few other families there are 

 several pairs of gonads and genital pores arranged serially along the 

 sides of each ray. 



The madreporite or madreporic plate is dorsal, 1 excentric, and 

 commonly single, yet in some multiradiate species there may be sev- 

 eral. It is an organ primarily for the purpose of eliminating the 

 excess of absorbed water from the ambulacral tubes and body-cavity. 



The sides of the rays and disk are generally supported by two 

 rows of marginal plates, usually larger or thicker than the other 

 plates, and commonly bearing special spines. They are called supra- 

 marginals or superomarginals and inf ramarginals or inferomarginals. 

 (See text-fig, i.) 



The upper row is sometimes much reduced, or obsolete, and rarely 

 both rows are lacking or rudimentary. These rows of plates belong 

 to the primary system of plates, and extend to the apical plate of 

 the rays, like the ambulacral and adambulacral rows. 



Plates are constantly added to these rows by the budding in of 

 new plates between the apical plate and the one next to it, the apical 

 plate being pushed farther outward and the ray lengthened at the 



1 In some paleozoic fossil starfishes it is said to be ventral, but it is not so 

 in any living species. The statement by Gregory (op. cit, p. 238, 1900) that it 

 is ventral in Asterina is erroneous. 



