288 



THE OCEAN WORLD. 



vegetarians. The organs of respiration of the Echinidae appear to 

 be certain flattened vesicles in the form of very delicate laminae, which 



adhere to the internal sur- 

 face of the walls of the 

 body, and float freely in the 

 liquid with which the vis- 

 ceral cavity is filled. These 

 organs, known as the in- 

 ternal branchias, are in com- 

 munication with the central 

 canal and ambulacral tubes. 

 The heart is spindle-shaped, 

 tapering above, swelling 

 below. There are two dis- 

 tinct vascular systems, one 

 intestinal, the other cuta- 

 neous. 



Their nervous system con- 

 sists of a ring, which sur- 

 rounds the gullet, and is 

 placed at a short distance 

 from the mouth. In this ring the nervous trunks have their origin. 

 In relation to the senses, that of touch is highly developed. Certain 

 branching tentacula, which surround the mouth, fashioned like 

 nippers, and the ambulacral tentacles, are its principal organs. They 

 appear to be altogether destitute of organs of sight. It has some- 

 times been argued that four or five red points at the summit of 

 the dorsal face are eyes ; but this opinion has not been maintained, 

 nor has any crystalline lens been found in these spots to justify it. 

 Captain de Conde states that he examined a sea-urchin with long 

 spears in a pool of water, which he tried to catch, when he saw 

 it direct its flight towards his hand, all its defences being erect. 

 Surprised at this manoeuvre, he tried to seize it from another quarter ; 

 its spines were instantly directed to the other side. " I have thought 

 from that time that the urchin saw me, and prepared to resist my 

 attack. In order, however, to satisfy myself whether or not the 

 movement in the water caused by my approach might have produced 

 the effect described, I repeated the experiment with greater caution. 



Fig. 118. Skeleton and Masticating Apparatus. 



