144 



ECHINODERMATA. 



its dense calcareous crust enclosing the viscera within its cavity, 

 while the locomotive apparatus is placed upon the external surface. 

 The mouth is a simple orifice in the shell placed at one extremity 

 of its axis, and through it, as represented in the figure, the points 

 of five singular teeth project externally ; while the anal aperture 

 occupies 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 prominent tubercle whereon it 

 moves. These numerous spines, therefore, which are essentially 

 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 facility. 

 But these wonderfully constructed animals are by no means con- 

 fined to this mode of progression ; for, impossible as it might 

 appear from their outward appearance, 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 pig. 52. 



issue, which protrud- 

 ing, as represented 

 in the figure (jig- 

 62), beyond the 

 points of the spines, 

 can be firmly fixed 

 to any smooth sur- 

 face, and, like the 

 suckers of Asterias, 

 become locomotive 

 agents. 



(185.) Holothu- 

 ridce. Having trac- 

 ed the developement 

 of the Echinodermata from the polypiform Encrinite to the globu- 

 lar Echinus, we now shall find them perceptibly approximate an 

 annulose or worm-like form. In the Holothuria (Jig. 70), the 



