308 ZOOPIIAGOUS MOLLUSCA — 



But the proboscis (Fig. 56), the organ by which this 

 work is effected, demands a more detailed description ; for 

 its mechanism is scarcely less wonderful than the analogous 



organ of the elephant. It is cylindrical, 

 Fig. 56. anc \ f considerable length, and when not 



in use is kept retracted within the body, 

 where it lies beyond the reach of injury. 

 The better to understand its structure, 

 we may represent it as being formed of 

 two flexible cylinders, one within the 

 other, and which are united at the upper 

 margin, so that, in drawing out the inte- 

 rior cylinder, we can only lengthen it at 

 the expense of the other ; and, on pushing it back again, 

 we, in shortening it, give corresponding extension to the 

 exterior, but the latter lengthens only on the upper side, 

 because it is fixed to the parietes of the head by its inferior 

 margin. Let us now add a number of longitudinal muscles, 

 all of them very much divided at both extremities : the 

 stripes of their internal or superior extremity are attached 

 to the parietes of the body, those of the opposite end all 

 along to the internal surface of the inner cylinder of the 

 proboscis ; and their action, consequently, is to draw this 

 cylinder and the whole proboscis inwards. When thus re- 

 tracted, a great part of the internal surface of the interior 

 cylinder makes part of the external surface of the exterior 

 cylinder, and it is just the contrary when the proboscis is 

 elongated and protruded. The protrusion of the inner cylin- 

 der by the unrolling of the exterior, or, which is the same 

 thing, the evolution of the proboscis, is effected by its own 

 peculiar annular muscles : these encircle it all its length, 

 and, by contracting in regular succession, they force it out 

 beyond the lips, in a manner perfectly similar to the evolu- 

 tion of the tentacula of the snail. There is, in particular, 

 one muscle, near the place where the exterior muscle is 

 attached to the head, which is stronger and more effective 

 in this operation than all the others. When extended, the 

 proboscis can be bent to all sides, and at any point, by the 

 action of the retractor muscles, parcels of them acting, while 

 others assume the place and office of antagonists. The Figs. 

 57, 58, and 59 will serve to illustrate this interesting mecha- 

 nism. In Fig. 57, the proboscis is retracted about a half: 

 the external cylinder (a) is seen enveloping a portion of the 

 inner (b), the end of which (c) is the end of the proboscis : 

 the muscles which draw it within the body (dd) are in a 

 state of contraction, and at e we see the great annular 



