424 Mr, Gray on the Sea Eggs, [Dec. 



furnished with numerous muscles for the purpose of moving 

 them in every direction. 



The manner in which the spines and plates enlarge has never 

 been satisfactorily explained. The plates appear to be placed 

 between two skins, and a small process of membrane seems 

 to extend between each of them. The spines evidently grow by 

 a disposition of matter placed under their outer edge, more espe- 

 cially at the apical extremity ; the matter is, perhaps, deposited 

 by the processes of the skin of the articulation being extended 

 up the longitudinal grooves with which these spines are always 

 furnished. Their manner of growth may easily be seen by 

 cutting down longitudinally the spines of Echinus mammilatus, 

 Earn, when the outer surfaces of each complete spine will be 

 distinctly visible in the form of a darker line, occasioned by the 

 outer edge being the hardest and most compact. 



The older naturalists paid very great attention to the group, 

 and several of them divided them systematically into " classes, 

 sections, and genera." Morton (1712), in his History of Nor- 

 thamptonshire, divided the fossil species found in that county 

 into three groups. Bryerius, in 1732, divided the Echini into 

 seven genera from the position of their mouth and vent : these 

 genera have all been adopted by Lamarck, but under other 

 names. Klein, two years after, published his natural disposition 

 o{*Echinodermata, where he divided them into nine sections, 

 containing twenty-two genera, placing them according to two 

 systems ; first, after the position of their vent, forming them into 

 three classes, called Anocystos, Catocystos, and Pleurocystos ; 

 secondly, with respect to the situation of the mouth, as Ernme- 

 sostomi and Apomesostomi. Van Phelsum, in 1714, extended 

 Klein's method by taking notice for the first time of the form 

 and extent of the Ambulacra, He divided them into the same 

 number of genera as Klein, but several of them were very differ- 

 ent from those of the latter. Leske, in 1778, published an addi- 

 tion to Klein, and he reduced the number of Klein's genera to 

 ten, which agree very nearly with the sections of his author, and 

 he adopted the prior names of Bryerius for his genera. Davilla, 

 in 1767, divided the Echini into six groups, according to the 

 general form of the shell, but these groups are very indefinite. 



Linneeus took no notice of the works of Klein or others, but 

 considered the whole of the group as one genus. MuUer divided 

 it into two, under the names of Echinus and Spatangus, 



The Echinida may be divided into two sections. 



1. Typical Group. — Body globular; mouth central, below; 

 jaws conical, projectile, with Jive acute teeth ; anus vertical, dor- 

 sal ; ambulacra complete, f or ming bands extending from the mouth 

 to the anus. 



The crustaceous covering of the body of these animals is 

 formed of twenty perpendicular bands, each formed of several 



