656 



ECHINODERMA— CLASS ASTEROIDEA. 



AsTEEOiDEA (Star-fish). 



The common cross-fish, Asterias rubens, is perhaps the commonest ex- 

 ample of an echinoderm, known to all visitors to the sea-side. Fig. 1 shows 

 it from the upper surface, one of the arms being slightly turned over so 

 as to show the ambulacral grooves with the tube-feet (P) on either aide. 

 Towards the middle of the body, at the junction of two of the rays, is seen 

 the madreporite (M). These two rays are known as the bivium, while the 

 other three rays form the trivium. A line drawn between the two rays 

 of the bivium, through the madreporite and down the central ray of the 

 trivium, would divide the star-fish into two equal and symmetrical portions. 

 In Fig. 2 the same animal is shown, also from the upper surface, but partly 

 dissected. In the ray marked IV. the rough spiny skin has been left 

 on, while it has been removed over the middle of the body from 

 portions of rays II. and III., and from the whole of the upper surface 

 of the ray marked I. In the middle is seen the stomach (S), which 

 sends into the rays the pouch-like extensions (D). At F part of the 

 roof of the stomach has been removed to show the folded arrangement of 

 its walls. Near the centre is the anus (As). From the stomach there pass 

 down in the rays blind, much-branched outgrowths (C), known as pyloric 

 caeca, which have been dissected out in the ray marked I. Underneath 

 these, on either side of each ray, are the generative glands (G), which are 

 seen in ray II. In rays I. and II. the central line of plates (Ao) separating 

 the ray is exposed. On either side of these there are shown in ray I. the 

 little swellings or ampullae (P) at the inner ends of the tube-feet. 



Fig. 3 is a diagram showing the arm of a star-fish in section. Here the 

 same organs may be seen, namely, the pyloric caeca (PC), the blind processes 

 from the stomach, the generative glands (G), which open to the exterior 



Fig 3. — DUGRAMATTC Oross Sfctton of 

 Star-fish Arm. Natural size. 



Fig. i.- 



-DlAGRAMATIC CROSS SkCTION OF OPHIURID 



Arm. Much enlarged. 



through a pore (GP) which is situated near the junction of two adjacent 

 rays, and the ampullae (Amp), from which the tube-feet (P) pass through the 

 ambulacral ossicles (AO). In the groove (A) on the under aide of the arm, 

 formed by the ambulacral ossicles and lying between the tube-feet, there are 

 shown the following structures : close to the surface, the radial nerve (N) ; 

 below this, the radial blood-vessel (B) ; and below this again, the radial water- 

 vessel (W). All of these give off side branches, those from the latter going 

 to the ampullae. The arm is encased in an external skeleton of irregular 

 limestone plates (SK) ; some of theae aupport spines (S), and others support 



