STKUCTUKE OF THE MOUTH. 



247 



Fig. 119. 



Edwards), will give a good idea of this curious organ when fully dis- 

 played ; and in fig. 119, B, the mechanism is exhibited by which its 

 protrusion and retraction are ac- 

 complished. The whole appa- 

 ratus is there seen to consist of 

 two muscular cylinders, placed 

 one within the other, but conti- 

 nuous at their upper margin (B) ; 

 or. to use a familiar illustra- 

 tion, the proboscis may be com- 

 pared to the finger of a glove 

 partially inverted. It is obvious 

 that, in this case, if the inner 

 cylinder be drawn inwards 

 that is, into the mouth, the 

 whole structure becomes short- 

 ened, until at last it is entirely 

 retracted into the oral cavity; 

 whereas, on the contrary, if the 

 outer tube is made to protrude, 

 it expands at the expense of the 



inner one, which is gradually Mouth of Goniada claviffera , 



drawn forwards. The internal 



surface of this remarkable proboscis, moreover, is variously modified in 

 its structure, so as to adapt it to the prehension of different kinds of 



prey. In Amphinome, for instance, 

 the orifice of the mouth is a thick, 

 fleshy, and callous circle (fig. 122, 

 6, c, d) f and the surface of the exserted 

 proboscis (c, d) is covered with deli- 

 cate transverse rugae, evidently so 

 arranged as to give tenacity to its 

 gripe. In Goniada it supports two 

 distinct sets of horny teeth, provided 

 for very different uses : one set, which 

 is exposed when the proboscis is un- 

 rolled to a very small extent, consists 

 of a series of linear horny plates 

 (fig. 119, A, d), and probably forms a 

 kind of file, or rather a scraper, 

 wherewith the animal excavates the 

 subterranean galleries in which it 

 lives. The other set does not make 

 its appearance till the proboscis is 



Fig. 120. 



Mouth of Phyllodoce laminosa. 



more completely expanded, and is evidently an instrument of pre- 



