176 THE COMMON EGG URCHIN : 



but we see in its structure a memory kept up of an 

 organ which is more fully developed in a kindred race. 

 The Star-fishes are closely connected with another 

 family, which differs chiefly in the more condensed form 

 of the body, and the more perfect solidification of its 

 shelly coat. I mean the SEA URCHINS, of which more 

 kinds than one frequently come up in the dredge. The 

 common Egg Urchin (Echinus sphcera), the largest and 

 best known of our British species, may be taken as an 

 example of the race. On comparing one of these Urchins 

 with a Star-fish, such as the Luidia, there is, at first 

 sight, so little outward similarity, that we should scarcely 

 suppose their close connection. But the more we exa- 

 mine them, the greater is the number of points which 

 we establish between them : the rows of sucking-feet 

 common to both ; the radiating lines in which all the 

 organs are disposed, and the correspondence between the 

 compartments into which the body is divided. There 

 remains, in the opposite scale, the difference of form. 

 But when we examine a series of Star-fishes, we find a 

 beautiful gradation of form, in which those with the 

 longest rays are insensibly connected with others which 

 are scarcely more radiated than some Urchins. There 

 are flat, discoid Urchins, and others of every degree of 

 convexity, till we come to the globose form of our Sea 

 Egg. We have also in the Sea Eggs, the representative 

 of the madreporiform tubercle of the Star-fishes, in a 

 state certainly much reduced, but sufficiently obvious. 

 So that, on the whole, the evidence in favour of the 

 close affinity of these two families of animals greatly 

 outweighs that against their connection. 



