STARFISH 



(asterias) 

 Classification: — Phylum Echinodermata. Class Asteroidea. 



The preserved starfish is rigid and the arms are immotile 

 but in the living animal the arms are flexible and capable of 

 movement in all directions. The starfish is even able to tvirn 

 completely over because of the mobility of its arms and in feed- 

 ing, the arms enfold the mussel or other animal that is being 

 devoured. The vivid colors characteristic of the living starfish 

 are also lost in the preserved specimen. 



The starfish will be studied as a representative animal in 

 which the organs are repeated, more or less perfectly, about a 

 central axis, and therefore it is said to be radially symmetrical. 

 During this study look for structures which prevent the starfish 

 from being a perfectly radial animal. 



As this kind of animal differs so much in structure from 

 the other animals you have studied it will be very important to 

 keep in mind the functions of the different organs, as functional 

 relations to other animals are more readily understood than its 

 structural relations. 



The entire study, including the dissection, must he made 

 from a single specimen. 



I. External Characters 



1. Observe that the body is composed of a central disc 

 from which the five rays or arms radiate. 



2. Examine the two surfaces of the disc. The mouth is 

 located in the center of one surface which is therefore named 

 the ora^ surface. Most of the ahoral surface of the disc is 

 covered with sharp spines except for a small rounded area be- 

 tween the bases of two rays. The spineless region is termed the 

 madreporite or sieve plate. 



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