NO. 19.] ECHINODERMS OF CONNECTICUT. 



97 



The digestive system of the sand-dollar (Plate XXIV, fig. 

 2) is essentially similar to that described for the key-hole urchin, 

 with the exception that the rectum opens to the exterior near the 

 edge of the disk, and is on that account longer than in the latter 

 species. 



The short esophagus leads from the large jaws surrounding 

 the mouth to the broad flat stomach. In life the stomach is 

 green in color, with a smooth inner border (Plate XXIV, fig. 2), 

 while its outer border is provided with many lobes of brownish 

 color. A narrow siphon extends along the inner border, as in 

 the key-hole urchin. The intestine nearly completes the wide 

 circle, of which the stomach forms a part, and then bends sharply 

 on itself to pass posteriorly, with many short bends, to the open- 

 ing near the edge of the disk. The opening is sometimes almost 

 exactly on the margin, but is more often placed near the edge of 

 the oral surface. In young individuals it is on the aboral 

 surface. 



HABITS 



The four species of Echinoids which are found in Connecticut 

 waters exhibit much diversity of habit, associated with differen- 

 tiations of structure. The two species belonging to the order 

 Regularia — the purple sea-urchin and green sea-urchin — are 

 provided with large spines for protection and locomotion, and live 

 free upon the surface of rocks, sand, shells, and other objects 

 found upon the bottom of the sea. Between tides they often 

 inhabit the tide pools in the rocks, hiding away in crevices and 

 beneath seaweeds, often clinging to the frond of the seaweed 

 itself and sometimes drawing it over the body as a protective 

 covering. The two species of clypeastroids — the sand-dollar 

 and the key-hole urchin,— on the other hand, are covered every- 

 where with extremely minute spines, and they live partly buried 

 beneath the surface of the sand. 



Locomotion. — The regular sea-urchins move about from 

 place to place both by the movements of the spines and by the 

 action of the very long tube-feet. Both sets of organs commonly 

 assist in the process. Certain species make the greatest use of 

 the spines in locomotion, while the tube-feet of other forms are 

 the most efficient. In exceptional cases the former are said 

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