COMMON STARFISH. 



195 



38. COMMON STARFISH (Asterias, or Asteracanthion rubens), 



Dissected so as to show its digestive and motor systems. 



ONE of the rays, the central ray of the trivium, has been cut short, and 

 more or less of the dorsal or anti-ambulacral integument removed from each 

 of the other four, and from the central disc. In the interradial space which 

 is opposite to the ray cut short, is seen the madreporite ; and a little to the 

 left of a line drawn along the axis of the central ray to the madreporic 

 tubercle, and near the centre of the disc is seen the small piece of dorsal 

 integument in which the anus opens. It lies on this aspect in the interradius, 

 to the left of the madreporite (see note, Plate xi.). From the intestine, and 

 close to the anus, arise two diverticula. They bear several irregular caecal 

 ampullae and reach a short way into two interradii, the interradius between 

 the central and left ray of the trivium and the interradius between the left 

 rays of the bivium and trivium respectively. The internal surface of these 

 diverticula is longitudinally plicated and they are probably highly extensile. 

 They are generally considered to be the homologues of the respiratory trees 

 of the Holothuroidea. The intestine itself cannot be seen. It is short and 

 arises from the pyloric division of the stomach. This pyloric division is 

 pentagonal in outline, and a single trunk may be seen to arise above each 

 angle of the pentagon. Each trunk enters the corresponding ray, and 

 divides into two branches which, with their foliaceous glandular ampullae, 

 fill up the greater part of the cavity of the ray. The saccular dilatations of 

 the anterior or cardiac division of the stomach are to be seen lying below 

 the trunks of origin of the complex caeca, and bulging for a short distance 

 into the cavities of the rays. These sacculi can be evaginated so as to 

 enclose the animals on which the Starfish feeds, e.g. young oysters, cockles, 

 &c., too large to be drawn into the disc. Each sacculus is retracted after 

 protrusion by a pair of muscles attached to the sides of the vertebral ridge 

 of the ambulacral groove. The Asteroidea are the only group of Echino- 

 dermata which possess a radial development of caeca to the digestive tract 

 as seen here. The two divisions of the arborescent caeca have been separated 

 in two of the rays to show the ampullae of the feet. These are arranged in 

 two symmetrical rows on either side of the vertebral ridge of the ambulacral 

 ossicles, thus corresponding to the arrangement of the pores seen in the 

 preceding preparation. 



On the ventral surface the mouth is to be seen placed centrally. The 

 ambulacral feet are variously contracted, some more, some less. They have 

 sucker-like ends, which are not supported by calcareous plates as they are 

 in Echinoidea and most Holothuroidea. In the left ray of the trivium the 

 feet are completely retracted, and in the left ray of the bivium the series of 



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