SEA-URCHIN. z6$ 



and along with five stout " braces " (rotulse) and five curved 

 "compasses" (radii) form "Aristotle's lantern," a complex 

 masticating apparatus, of whose history we know little. It 

 surrounds the pharynx, and is swayed about and otherwise 

 moved by muscles, many of which are attached to five 

 beams which project inward from the margin of the shell 

 and form a "girdle" round about the mouth. 



Int 



FIG. 137. Dissection of sea urchin. 



M. at the lower pole is the mouth ; M. at the upper pole is the madreporic 

 plate; T. T., one of the large tentacular tube-feet around the mouth; 

 S.G., a skin-gill; ST., a standard or perignath ; AL., an alveolus; 

 R.V., a radial vessel, with ampullae (A.); intestine (Int.) fixed by 

 mesenteries; /*., a pedicellaria ; G., a gonad : SP., spines; T.F., 

 tube-feet. 



As in other Echinoderms, the skeleton of lime is meso- 

 dermic. The shell is covered externally by a delicate 

 ciliated ectoderm, beneath which, in a thin layer of con- 

 nective tissue, there is a network of nerve fibres, and some 

 ganglion cells. Internally, there is another thin layer of 

 connective tissue, and a ciliated epithelium lining the 

 body cavity. The skeleton grows by the formation of 



