ECHINUS. 203 



process.* From the shortness of these feet, 

 notwithstanding their great number, the advance 

 which this animal can make in any particular 

 direction is excessively slow. 



Besides this movement of creeping, the Asterias 

 is capable of bending and unbending each of its 

 rays; actions, however, which it can perform but 

 very slowly, and not to an extent sufficient to 

 accomplish its removal from one place to ano- 

 ther, t 



The skeleton of the Echinus or sea-urchin, 

 (Fig. 91), is still more artificially framed than 

 that of the Asterias. It has a spheroidal form, 

 like that of an orange ; the calcareous material 



* The mechanism by which the feet are protruded and re- 

 tracted is illustrated by the diagram, Fig. 97, which exhibits the 

 bladders connected with them, in different states of distention 

 and contraction. Fig. 96 shows the upper side of the ambu- 

 lacra, and of the bladders connected with the feet. Dr. Grant, 

 from some observations which he made on the structure of the 

 cilia of the Beroe pileus, is led to suspect that the rapid vibra- 

 tions of these singular organs in the lowest animals may depend 

 on the undulations of water conveyed through elastic tubes 

 along their bases, in a manner resembling the injection of the 

 tubular tentacula of Actiniae and Asterise. If this conjecture 

 were verified, he remarks, one of the most remarkable phenomena 

 of animal motion, though one of the most frequent, would lose 

 much of its present marvellous character. 



t In addition to these larger tubes, there exists also a smaller set, 

 which pierce the skin in different places, and are channels for 

 the absorption of the water used in respiration. These 1 shall 

 have occasion to notice more particularly hereafter. 



