STARFISH, SEA-URCHINS, AND SEA-CUCUMBERS. 41 



The mouth (Fig. 37, m) is capacious, opening by a short oesophagus 

 into a capacious stomach (Fig. 37, *) with thin distensible walls, and 

 sending a long lobe or sac (Fig. 37, 1) into the base of each arm; each 

 sac is bound down by two muscles attached to the median ridge lying 

 between the two rows of water sacs (ampullae, see also Fig. 38). The 

 stomach ends in a short intestine, the limits between the two not dis- 

 tinctly seen. Appended to the intestine are the " creca" or " liver" 

 (Fig. 37, b), consisting of two long tree-like masses formed of dense 

 branches of from four to six pear shaped follicles, connecting by a 

 short tube with the main stem. The two main tubes 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 water vascular sj^stem consists of the madreporic body, the 

 " stone canal " (Fig. 37, f), the ring 

 or circumoral canal (>), and the ra- 

 dial vessels (v) ending in the water- 

 sacs (am) and the feet. The stone- 

 canal begins at the outer and under 

 side of the sieve-like madreporic body, 

 passing directly forward and down- 

 ward in a sinuous course to the edge 

 of the mouth. The sea water in part 

 enters the body-cavity through the fis- 

 sures 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. 38, b) passes directly into 

 the water-vascular ring (Fig. 38), and 

 then into the ten Polian vesicles and 

 the five radial canals, whence it is con- 

 veyed to each water-sac or ampulla 

 (Fig. 38, c; compare also Fig. 37). 

 These pear-shaped water-sacs, when 

 contracted, are supposed to press the 

 water into the long slender suckers 

 or anibulacral feet, which are dis- 

 tended, elongated, and, -by a sucker 

 like arrangement at the end, act in 

 conjunction with the others to warp FlG ' 39 - Encrinus or Stone-lily. 

 or pull the starfish along. Besides locomotion, the ambulacral feet 

 serve for respiration and perception 



