MUSEUM OF COMPAEATIVE ZOOLOGY. 13 



other plates. It is possible, in this connection, that the large spines of 

 the terminals may also serve as structures for protection of the tip of 

 the ray. Animals small enough to take the young starfish for food 

 might well hesitate before eatiug an animal bristling with the sharp 

 needles of the rays of the young Asterias. While such an explanation 

 may be probable, it also seems not unreasonable that these enormously 

 large spines point to features of the ancestors of the starfish, and have a 

 morphological significance. 



It is likewise to be noted, that in many genera of Asteroidea the 

 primarily formed spines are relatively of great size. This is also true 

 of many Ophiuran genera, as Ophiothrix and Ophiocoma. Among 

 Echinoids, Echinarachnius and several others have large primary 

 spines, and in the former genus, as I have already pointed out, these 

 spines appear very early in the embryonic history. Their existence in 

 these groups would seem to indicate a morphological meaning. 



It seems to me a significant fact, however, that while in Ophiothrix 

 the spines of the terminals bear the form of large hooks, as I have 

 noticed in an Ophiothrix from Santa Barbara, Amphiura in the pouch of 

 its mother is destitute of terminal primary spines. The spines of the 

 larval Asterias are larger than those of the larval Asterina. 



The general character of the above plates are as follows : — 



Flat, discoid. — The interradials, connectives, dorsocentral, dorsals, 

 and dorsolaterals are flat, discoid, or cylindrical in shape. 



Massive, quadrangxdar. — The interambulacrals are quadrangular and 

 massive. 



Curved, crescentic. — The marginals and genitals are crescentic or 

 curved, or more or less bent out of a plane surface. 



Elongated in the plane of first calcification. — The ambulacra! rafters 

 are elongated in the plane of the first calcification, and have the form 

 of bars or beams, rather than flat plates. 



Elongated at right angles to plane of first calcification. — The spines 

 are elongated in the plane opposite that in which the first calcification 

 occurs. . 



Cap-shaped. — The terminals are cap-shaped. 



Tubular. — The calcification of the stone-canal is tubular and mul- 

 tiple, or originating from several centres. 



Double calcification in same organ. — The calcification of the pedicel- 

 laria3 is double from the first. 



The above classification is not intended as a hard and fast division, 

 but only as a means of roughly separating the plates from each other. 



