By the Rev. T. A. Preston. 



95 



case. It really has two distinct larval forms, one of which — until 

 its true nature was known — was called a Pluieus, being naturally- 

 considered to be a distinct species. In its earliest stages the young 

 echinus consists of an oval body coated with moveable hairs, which 

 enables the animal to swim about freely in the water. In process 

 of time the alimentary canal, with its mouth, stomach, &c, is formed 

 from side to side of the young echinus, the hairs gradually dropping 

 off till only a band round the animal remains. These hairs then 

 gradually grow out into long processes, and a curiously complex 

 skeleton is formed. This is considered to be the first larva, and was 

 formerly called a pluteus, as mentioned above. The second process 

 is very extraordinary. At one side of the stomach of the pluteus 

 springs a bud, which gradually grows into an echinus with spines, 

 &c, all but a stomach, and this it supplies by taking that of the 

 pluteus, discarding all the rest, which consequently dies. 



It remains to say a few words with respect to the animals to 

 which it is allied, for it forms a link in a very interesting series of 

 animals, grouped under the term Echinodermata. Echini are not 

 always melon-shaped. Some species are very flat, as if they had 

 been compressed by a weight placed on them, nor are they always 

 circular, portions of the inter-ambulacral areas are occasionally 

 wanting. If, then, we replace the calcareous corona by a tough skin, 

 in which calcareous particles are imbedded, and remove the greater 

 portion of the inter-ambulacral areas, we very nearly get a star-fish. 

 If, again, we replace the corona of the echinus by a skin with a few 

 calcareous particles imbedded in it, lengthen out the distance between 

 the two poles, and enlarge the gills round the mouth, we get an 

 approximate Holothurea or sea-cucumber. If we take a star-fish, 

 replacing the calcareous plates and making the arms really distinct 

 from the body, we get Sand-stars. And, finally, if we place a 

 Sand-star on the top of a stalk, we practically get a Stone-lily. The 

 different forms are, of course, accompanied by other internal changes, 

 by which evolutionists satisfy themselves that they can trace the 

 evolution of one species from another. The dental apparatus, called 

 Aristotle's Lantern, is found complete in the Echinus, only in a rudi- 

 mentary form in the Sea-cucumber and is wanting in all the others. 



