SEA-URCHINS AND SEA-CUCUMBERS 333 



ter purchase, or power, than they could have had if they 

 had been inserted into the slender stem itself. 



The tubercles on the shell show a structure which cor- 

 responds with this. They are very minute; but each of 

 them is regularly formed, and is crowned with its little 

 polished nipple, on which, as I have said, the spine 

 works, as by a ball-and-socket joint. These are arranged 

 with perfect regularity in quincunx, and by close exami- 

 nation you will see that each is enclosed in a little area 

 formed by a very low and narrow ridge of the shell, which 

 makes a network. On the lateral portions of the under 

 surface the meshes of this net are particularly conspicu- 

 ous, and we see that they constitute shallow hexagonal 

 cells, in the midst of which is seated the tubercle; yet not 

 in the exact centre either, but nearer the front than the 

 back of the area enclosed. 



Now this elevated ridge affords, doubtless, the inser- 

 tion of the other end of the muscles that move the spine; 

 the ridge giving a better purchase than a flat surface, as 

 the keel on the breastbone of birds is deep in proportion 

 to the vigor of the muscles used for flight. And, surely, 

 the apparently trivial fact that the space behind the tu- 

 bercle is greater than that in front is not without signifi- 

 cance, since it implies a thicker muscle at that part, which 

 accords with the circumstance that such would be the in- 

 sertion of the muscle- band whose contraction produces the 

 outward stroke by which the sand is farced away from 

 the bed. 



But what is the need of so much care being bestowed 

 upon the separate motion of these thousands of hair-like 

 spines, that each individual one should have a special 

 structure with special muscles for its individual move- 



