3i° 
ECHINODERMS. 
developed. The spines of sea-urchins also serve as organs of protection; but their 
efficacy varies much in different forms. For instance, Diadema setosum has fine 
sharp spines, 8 or 10 inches long, which prick one almost before one can see them, and 
can pierce the stoutest boot; their danger being increased by the gregarious habit 
of the animals. Some sea-urchins have poison-glands attached to their spines. 
It is the smaller spines that are protective, and they are placed for this purpose 
near the main openings and organs of the body, such as the vent, genital pores, 
and eyes; they also protect the ambulacra, and bases of the larger spines. A 
Porocidaris feeling itself free from danger, in well aerated water, walks from 
one side to the other, doubtless in search of food; its ambulacral tentacles being 
FIDDLE HEART-URCHIN, MOVING OVER THE SAND TOWARDS THE LEFT (nat. size). 
stretched out as feelers, and its long spines moving as described. The smaller 
spines are depressed to permit of the free movement of the larger ones, and those 
of the ambulacra raised to permit the extension of the tube-feet. If one slightly 
wounds the animal when thus expanded, the larger spines immediately stiffen on 
their tubercles, while all the smaller spines depress themselves, each over the 
organ that it is destined to protect. Though the tube-feet may not be used for 
locomotion, they are put to another useful purpose. If a Strongylocentrotus be 
placed in a tank with some dead shells or similar objects, it will raise them on to 
its back, and hold them there by means of the tube-feet, as a kind of concealment. 
Some sea-urchins cover themselves all over in this way with bits of seaweed, shell, 
and small pebbles, and so move about unobserved. Other sea-urchins do not move 
