STRUCTURE OF COMMON STAR-FISH. 181 



of the aboral disk. The anus (Fig. 125, a) is minute and 

 difficult to detect, being situated between the short spines,, 

 and is evidently not used in the expulsion of fecal matter 

 unless the urinary secretions, if there be such, pass out of 

 it. Tt would seem as if the opening were rudimentary and 

 that the star-fish had descended from Echinoderms like the 

 Crinoids, in which there is a well-marked external terminal 

 opening of the digestive tract. Appended to the intestine 

 are the " cceca " or " liver " (Fig. 125, b), consisting of two 

 long, tree-like masses formed of dense branches of from 

 four to six pear-shaped follicles, connecting by a short duct 

 with the main stem. The two main ducts unite to form a 

 short common opening into the intestine. The cceca are 

 usually dark, livid green, and secrete a bitter digestive 

 fluid, representing probably the bile of the higher animals. 



The star-fish is bisexual, but the reproductive glands are 

 much alike, the sexes only being distinguishable by a micro- 

 scopic examination of the glands. The ovaries (Fig. 125, 0) 

 are long racemose bodies lying along each side of the in- 

 terior of the arms, and the eggs are said to pass out by a 

 short narrow oviduct (or) through an opening between two 

 plates on each side of the base of the arms, the opening be- 

 ing small and difficult to detect. 



The water-vascular system consists of the madreporic 

 body, the " stone-canal " (Fig. 125, t), the ringorcircumoral 

 canal (vr), and the radial vessels (v) ending in the water- 

 sacs (am) and ambulacra! feet. The stone-canal begins 

 at the outer and under side of the sieve-like madreporic 

 body, passing directly forward and downward in a simious 

 course to the under side of the circumoral plates. The 

 madreporic body (nib) is externally seen to be perforated by 

 linear apertures radiating and subdividing toward the pe- 

 riphery. The sea-water in part enters the body-cavity 

 through the fissures in the madreporic body, while most of 

 it enters the stone-canal, 'which is a slender tube scarcely 

 one fourth the diameter of the entire madreporic body. 

 The water entering the stone-canal (Fig. 125, t) passes di- 

 rectly into the water-vascular ring (Fig. 125) and then into 

 the ten Polian vesicles and the five radial canals, whence 



