CLASS III. ECHINODEKMATA. 



THIS word, which signifies " hedgehog- skinned," 

 sufficiently expresses the character that is most con- 

 spicuous in these animals, and which is so gene- 

 rally prevalent as to serve in most cases for their 

 identification. They are clothed with prickles ; 

 and these are either conical, sharp-pointed warts, 

 which are immoveable, studding the leathery skin, 

 or they are symmetrical spines variously shaped 

 and sculptured, usually jointed upon the surface of 

 shelly plates, so as to be capable of motion to a 

 certain limited extent. 



The form of the animals of this group is also 

 characteristic; though it varies much in appear- 

 ance, a moment's observation shows that there is 

 a common principle in the whole, which reduces all 

 the varieties to modifications of one model. That 

 model is a star with five radiating points or rays, 

 such as we see in the most simple condition in the 

 Sand-star (Ophiurd), a central body round and flat, 

 with five long taper rays set around the edge, like 

 the tails of snakes, diverging in as many directions. 

 Sometimes these rays consist each of two filaments j 

 springing from the same base, and then we have a 

 Comatula ; or the ray may divide, and subdivide, | 

 and subdivide again, to a high degree of ramifica- 

 tion, until the terminations are immensely nume- 

 rous and of hair-like fineness, and thus we have 

 a Medusa's Head (Astrophyton). The rays may 

 become so broad at the base as to merge into 

 one another, and we have the common Star-fish 

 ( Uraster) ; they may be more numerous than five, 



