122 THE MICROSCOPIST IN BERMUDA. 



same conditions remain, there can be no material improvement 

 made on the forms of animal life which nature iirst provides to 

 meet those conditions. 



In searching among the rocks at lowest tide, and in protected 

 coves of the sea-shore, I found, hidden away in little round 

 excavations just large enough to hold them, some fine specimens 

 of the large-spined sea-urchins or Echini. It seemed to me that 

 they had hollowed out and enlarged these secure homes for 

 themselves as they grew, for I never could get them out without 

 breaking up the rock in which I found them. They have strong 

 spines all over their round shell, and strong muscles to move 

 them ; still it would seem to be a Herculean task for them to ex- 

 cavate into the solid rock. Yet there they were in their little 

 round holes, so securely braced in that after pricking my fingers 

 most unmercifully in trying to get them out, I soon gave it up 

 and resorted to the hammer and cold-chisel. 



The Echinus in its development is about midway among the 

 invertebrates. It has a very fair digestive apparatus, and a little 

 show for a nervous system. Its great peculiarity is that all its 

 parts are arranged in divisions of five. Its mouth has five jaws, 

 and five teeth that meet together in one point. It has five little 

 red eyes on the opposite side of the body from the mouth. Five 

 similar segments make up the shell in which it lives ; and the 

 plates of which they are formed are five-sided. The shell in- 

 creases in size by these pentagonal plates enlarging at the joints. 



The test or shell of the Echinus bears three different kinds of 

 members. The first are the spines. Of course these differ in 

 size, shape, and function in the different families. For instance 

 in the Spatangoids, which live in the sand, they are spoon or 

 spud shaped, for the purpose of digging away the sand and then 

 covering tlie animal with it. But in the kind we are describing, 

 the Echidna, they are strong and conical, running to a point, 

 for the purpose of wearing away the rock. They are as beauti- 

 fully fluted as an Ionic column. Each one has a socket at the 

 base which fits on a little protuberance on the shell, forming a 

 perfect ball and socket joint ; and a set of external muscles 

 moves the spine in every direction. The internal structure of 



