216 



ANNELIDA. 



worms (Goniada a chevrons, Milne Edwards), will give a good 

 idea of this curious organ when fully displayed; and in Jig. 90, 

 B, the mechanism is exhibited by which its protrusion and re- 

 traction are accomplished. The whole apparatus is there seen to 

 consist of two muscular cylinders, placed one within the other, but 

 continuous at their upper margin (B), or, to use a familiar illustra- 

 tion, the proboscis may be compared to the finger of a glove 

 partially inverted ; it is obvious that in this case, if the inner cylin- 

 der be drawn inwards, that is, into the mouth, the whole 

 structure becomes shortened, 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 drawn forwards. The internal surface of this 

 remarkable proboscis is, moreover, variously modified in its struc- 

 ture, 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- 93, b, c, d), and the surface of the 

 exserted proboscis is covered with delicate transverse rugae, evi- 

 dently so arranged as to give tenacity to its gripe. In Goniada it 

 supports two distinct sets of horny teeth, provided for very differ- 

 ent 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, (Jig. 90, A, d 9 ) and probably forms a kind of file, or ra- 

 ther a scraper, with which the 

 animal excavates the subterra- 

 nean galleries in which it lives. 

 The other set does not make its 

 appearance till the proboscis is 

 more completely expanded, and 

 is evidently an instrument of pre- 

 hension, formed by two horny 

 hooks (Jig. 90, B, a, b) placed 

 upon an elevated ridge near the 

 entrance of the oesophagus, so as 

 to take a secure hold of any vic- 

 tim seized by this curious mouth. 

 In Phyllodoce laminosa the 

 teeth form a circle of semi-carti- 

 laginous beads, encompassing the 

 extremity of the proboscis when 

 that organ is pushed out to its 



Fig.91. 



