3278 



ECHINODERMS 



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 stretched out as 



PHIAL-SHAPED POURTALESIA; TEST WITH SPINES REMOVED. 

 (Enlarged four times.) 



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 



LEATHER URCHIN. 

 (Two-thirds natural size.) 



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 



