OPHIOPHOLIS ACULEATA Linnaeus 



Material. Ophiopholis aculeata is very common in tide- 

 pools along the Atlantic Coast north of Cape Cod. Dissection 

 of the internal organs is not satisfactory and the brittle star 

 should be used only for comparison with other classes of Echino- 

 dermata. Specimens preserved in alcohol will therefore suffice. 

 The study may be, however, supplemented by an examination of 

 cross-sections through an arm and the disc. For this purpose 

 very small specimens should be preserved in a fixing fluid and 

 decalcified before sectioning. 



Descriptive Part 



Ophiopholis aculeata is a representative of the Class Ophiu- 

 roidea. While its general plan of structure is similar to that of 

 the starfish, there are many important differences in structure. 

 The radial symmetry is apparent in the whole organization of 

 the brittle star, but the bilateral symmetry is still in evidence 

 from the position of the madreporic plate situated on the oral 

 or ventral surface of the animal. The body, especially on the 

 aboral or dorsal surface of the animal, is sharply differentiated 

 into a central disc and five narrow, long arms. The ossicles of the 

 arms are so articulated as to allow an extensive lateral motion, 

 whereas the up-and-downward motion is very slight. Owing to 

 the fact that the arms are narrow their visible bases do not 

 occupy the entire periphery of the central disc but are separated 

 from each other by those regions of the central disc, which be- 

 long to the interradii. The dermal skeleton of the aboral surface 

 of the central disc is composed of a system of distinct plates, 

 while the areas between these plates are covered with short 



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