ANNULOIDA I ECHINODERMATA. 



drawn apart. The upper surface of a star-fish corresponds 

 to the combined inter-ambulacral areas of an Echinus, and 

 exhibits the aperture of the anus (when present), and the 

 " madreporiform tubercle," which is situated near the angle 

 between two rays. The inferior or ventral surface corresponds 

 to the ambulacral areas of an Echinus, and exhibits the mouth 

 and ambulacral grooves. 



The mouth is central in position, and is not provided with 

 teeth ; it leads, by a short gullet, into a large stomach, from 

 which a pair of sacculated diverticula are prolonged into each 

 ray. A distinct intestine and anus may, or may not, be pre- 

 sent but the anus is sometimes wanting (in the genera, Astro- 

 pecten, Ctenodiscus, and Luidia}. 



The ambulacral system is essentially the same as in the 

 Echinoidea, and is connected with the exterior by means of the 

 "madreporiform tubercle," or "nucleus," two, three, or more 

 of these being occasionally present. The ambulacral tube-feet 

 are arranged in two or four rows, along grooves in the under 

 surface of the arms. Each ambulacral groove is continued along 

 the lower surface of one of the arms, tapering gradually towards 

 the extremity of the latter. The floor of each groove is con- 

 stituted by a double row of minute calcareous pieces the 

 " ambulacral ossicles" which are movably articulated to one 

 another at their inner ends. At the bottom of each groove-is 

 lodged one of the radiating 

 canals of the water- vascular 

 system or ambulacral system, 

 from which are given orT 

 the rows of suctorial feet, or 

 " tube-feet." It follows from 

 this that the radiating vessels 

 of the ambulacral system 

 are outside the chain of 

 ambulacral ossicles, so that 

 these latter are to be regarded 

 as an internal skeleton, and 

 they do not correspond with 

 any part of the skeleton of 

 Echinoids* at least they do 

 not correspond with the per- 

 forated ambulacral plates of 

 the Sea-urchins. The am- 



ig- 59- Diagram of a star-fish (Gonias- 

 ter), showing the under surface, with the 

 mouth and ambulacral grooves, a Am- 

 bulacral ossicles, with the ambulacral 

 pores between them, b Adambulacral 

 plates, bounding the ambulacral grooves ; 

 m Marginal plates (wanting in many 

 species) ; o Oral plates, placed at the 

 angles of the mouth. 



* The structures in the Echinus, which are truly homologous with the 

 ambulacral ossicles of the Asteroidea and Ophiuroidea, are the so-called 

 " auriculae." 



