HOLOTHUKIDyE. 



169 



Echinus esculentus. 



externally ; while the anal aperture is situated at the opposite pole of 

 the sphere. The instruments of locomotion occupy the entire superficies 

 of the shell, and consist of two distinct sets of organs adapted to different 

 uses. The first consists of a multitude of sharp purple spines, every one 

 of which is articulated 



to a distinct and promi- Fig. 85 



nent tubercle whereon 

 it moves. These nu- 

 merous spines, there- 

 fore, which are essen- 

 tially similar in their 

 office to those we have 

 already described in 

 Scutella, differing only 

 in proportionate size, 

 are so many inflexible 

 legs, upon which the 

 Echinus rolls itself from 

 place to place, or by 

 their assistance it can 

 bury itself in the sand 

 with the greatest faci- 

 lity. But these won- 

 derfully constructed animals are by no means confined to this mode of 

 progression ; for, impossible as it might appear from their outward ap- 

 pearance, they are able to climb rocks in search of food, and thus destroy 

 the corallines and shell-fish upon which they principally feed. In order 

 to effect this, we find the shell perforated with ten rows of small orifices, 

 so disposed as to form five pairs of ambulacra extending from one pole to 

 the other : through these apertures a system of long suckers is made to 

 issue, which, protruding (as represented in fig. 85) beyond the points 

 of the spines, can be firmly fixed to any smooth surface, and, like the 

 suckers of Asterias, become locomotive agents. 



(442.) HOLOTHUBID^. Having traced the development of theEchino- 

 dermata from the polypiform Encrinite to the globular Echinus., we now 

 shall find them perceptibly approximate an annulose or worm-like form. 

 In the Holoihuria (fig. 100), the commencement of this change is per- 

 ceptible : instead of being composed of hard, calcareous pieces, the in- 

 teguments of the body now become soft and irritable, a few thin laminaB 

 of earthy matter around the mouth being the only vestiges of the shell, 

 and the spines, of course, are no longer met with ; the suckers, however, 

 remain, and, when protruded through innumerable apertures distributed 

 over the surface of the body, they still form the principal instruments 

 of progression. 



(443.) FISTULARID^. At length, in the last division of the class, even 



