ASTERIAS VULGARIS (A Starfish). 



Phylum VIII, ECHINODERMATA; class i, ASTEROIDEA; order 2, 



CRYPTOZONIA. 



HABITAT. The common starfish which forms the subject of this 

 study is found along the Atlantic coast. Many other species occur in 

 temperate and tropical seas the world over. They are sluggish crea- 

 tures, clinging to rocks and timbers or crawling slowly about on the 

 sea bottom, often in large numbers, feeding on the various mollusks 

 they encounter. In color A. mdgaris varies considerably, shades of 

 brown and reddish-yellow predominating. Occasionally a specimen is 

 bright purple. 



Technical Note. The student will find a dried specimen best for studying 

 the external features. A hand lens will be useful in making out 

 some of the structural details. 



ABORAL SURFACE. This is the more convex of the two surfaces. 

 In studying it, note the following: 



a) The body or disc, pentagonal in shape, with its five radiating 



arms. 



b) The madreporite, a circular mushroom-like plate, light in color, 



near the junction of two of the arms. Examine it with the lens. 

 The presence of this plate interferes with the radial symmetry 

 of the animal. By drawing a line through its center and through 

 the middle of the opposite arm we may demonstrate a bilateral 

 symmetry, 



c) The skeleton of the animal, made up of calcareous plates with 



projecting spines. These plates are not joined one to the other 

 except by muscle and connective tissue, an arrangement which 

 gives some flexibility to the arms. In the dried specimen, of 

 course, the whole structure is rigid. The separate plates will be 

 demonstrated later. 



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