SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 133 



certainly in the case of animals at least, is its training in 

 the perception of form, and one should strive to learn not 

 merely to count or measure spines, but to perceive those 

 real differences of form which are often so difficult to 

 explain in words, but which constitute the true distinctions 

 between species. The brittle-stars are especially adapted 

 for exercises of this kind, and before you begin to study 

 the minute details of structure, you should strive to acquire 

 an exact knowledge of the general form. It is an interesting 

 if somewhat humiliating experience to look at a brittle-star 

 for a few minutes, then to cover it up and endeavour either 

 to draw or to even merely visualise the specimen, and then 

 compare your mental image or your sketch with the real 

 object. Both generally leave much to be desired in the way 

 of precision. 



There are a considerable number of other brittle-stars, or 

 sand-stars, which may occur between tide-marks, especially 

 after storms. One which occurs there freely in the living 

 condition, but is liable to be overlooked on account of its 

 small size, is Amphiura elegans. It should be looked for 

 under stones, and does not usually exceed one inch to one 

 and a half inches in length. The colours are sober and 

 inconspicuous, and the creature may be recognised by its 

 round disc with well-marked radial shields, and the slender 

 arms whose side plates bear three to four inconspicuous 

 spines. There are three mouth-papillae on either side of 

 the mouth-clefts. After storms, or among the wreckage at 

 most seasons of the year, the common sand-stars Ophiura 

 lacertosa and 0. alUda are to be found. They can be 

 recognised by the fact that the disc is cleft at the origin of 

 the arms, the clefts being fringed by papillae. In the larger, 

 0. lacertosa, these papillae are ten to twelve in number, 

 while in the smaller, 0. albida, they number about thirty. 

 The arms bear only minute spines, which are so closely 

 adpressed to the sides of the arms that they are not seen on 

 casual view. The disc is completely covered with scales. 

 The sand-stars occur perhaps most frequently in the 

 skeletonised condition, high up on the shore, and are then 

 admirable subjects for the study of the Ophiuroid skeleton 

 (see Fig. 43). 



The Ophiuroids in general offer many interesting points 



