ASTERIDEA. 495 



very natural group formed by these orders. A more or 

 less complete calcareous skeleton is always developed 

 within the Echinoderms, resembling that of the Actinozoa, 

 not only in this respect, but also in consisting of detached 

 spicula. In this form the skeleton remains in the Holo- 

 thuridea, but in the other Uchinoderms, the spicula coalesce 

 into networks, which may become consolidated into dense 

 plates by additional deposits. It is by the different shape 

 and arrangement of these plates that the diversity ex- 

 hibited by the skeletons of different Echinodemata is 

 produced." 



Asteridea. Star-fishes have been divided according to 

 the mode of locomotion into Spinigrades, moving by 

 means of spines ; Cirrhigrades, by suckers ; and Pinni- 

 grades, by fins or pinnae. Of the last-named division we 

 have only one British genus, Comatula. At the very 

 extremity of each ray is an organ like an eye, having 

 spinous appendages, which are termed the eyelids. It is 

 doubtful, however, whether these parts have really any 

 visual endowment ; no proof of their possessing the 

 faculty of sight has ever been advanced, and, from what 

 we know of the nature of this sense generally in the 

 lowe'st forms of animal life, we should be disposed to con- 

 sider that the organs in question must serve some other as 

 yet unknown purpose. 



The species Ophiura and Ophiocoma, Plate IV. Nos. 88 

 and 91, may be easily recognised by the great length and 

 tenuity of their rays, and their excessive fragility. The 

 whole surface, both of disc and rays, is covered by scales, 

 which are so closely approximated as to give an almost 

 perfectly smooth surface. These scales are arranged in 

 definite and often in very beautiful patterns, and in some 

 species the primary scales are edged or encircled by series 

 of circular bosses or tubercles, giving a reticulated appear- 

 ance to the disc and rays. Ophicoma rosula has its spines 

 tipped with curious anchor-shaped processes, which are 

 supposed to facilitate the motion of the creature. In 

 Ophiura they are very short, and not apparent without 

 careful inspection ; while in Ophiocoma they are so long 

 as to give quite a bristly, sinous appearance to the animal, 

 being sometimes, ' in fact, very much longer than the 



