THE ASTERIDEA. 477 



The spines are more or less movably united with the 

 ossicula, but there are no such regular joints as are met with 

 in the Echhiidea. The pedicellarice are supported upon 

 short, flexible peduncles. The skeleton of each consists of 

 two blades articulated with a basal piece. From the centre 

 of this, very strong adductor muscles proceed to the inner 

 faces of the blades, and weaker fibres, attached to the exterior 

 and to the outer faces of the bases of the blades, act as 

 divaricators. 



The gullet opens into a wide stomach produced into five 

 large cardiac sacs, the walls of which are subdivided into 

 many sacculi. Each cardiac sac is radial in position, and may 

 extend a short way into the cavity of the arm, to which it 

 corresponds. On the aboral side of these sacs the alimentary 

 canal suddenly narrows, and then dilates again into a shallow, 

 but wide, pentagonal pyloric sac, the angles of which are 

 produced into five tubes. Each of these passes along the 

 middle of the aboral face of a ray, and divides into two 

 branches, which run parallel with one another through half 

 or two-thirds the length of the ray, and end blindly. The 

 branches give off numerous caecal dilatations, arranged in 

 pairs on opposite sides, and these hang down into the cavity 

 of the ray. The edges of the pentagonal pyloric sac, and the 

 aboral faces of its sacculated branches, are connected by 

 mesenteric folds with the aboral perisoma. The oral faces of 

 the cardiac sacs are similarly connected by pairs of mesenteric 

 folds with the sides of the corresponding series of ambulacral 

 ossicles. The aboral face of the pyloric sac presents an aper- 

 ture closed by projecting valvular folds, which leads into the 

 short tubular intestine. The latter terminates in a minute 

 anal pore, situated nearly in the centre of the aboral face of 

 the bodv. The intestine receives the duct of a caecum divided 

 into two main branches, each of which has many minor sub- 

 divisions, If the animal, having its mouth downward, is di- 

 vided into two halves, by a vertical plane passing through the 

 mouth, the central point of the aboral face, the madreporic 

 tubercle, and the middle line of the ray opposite to the tu- 

 bercle ; and if this ray is anterior, then the anus opens into 

 the left posterior interradial space, and the caeca lie partly in 

 this and partly in the left anterior interradial space. 



The nervous ' and vascular systems of the Star-fish are so 



1 See Wilson, " The Nervous System of the Asterida" (" Transactions of 

 the Linnasan Society," 1862), and the later contributions of Prof. Teuscher, cited 

 below. 



