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406 THE POPULAR SCIENCE MONTHLY 



the posterior extremities are expanded, the siphons (Fig. 10, e and d) 

 can be carried outside into the water ; at the same time the access of 

 the two siphons, and consequently the entrance and exit of water, can 

 be more or less hindered. As we have seen, the mantle is prolonged 

 in the direction of the shell in an appendage which extends over the 

 two sides of the dorsal surface of the valves (Fig. 1, a, and Fig. 2, #), 

 the central portion of which forms a swelling of considerable thickness, 

 composed of various anatomical elements ; beneath the epidermis the 

 tissues are partly of vesicular and partly of membranous character, 

 which, through inherent powers of swelling and hardening under the 

 action of the blood, serve a purpose in operating the movement of the 

 valves. 



To explain the physiological role of this organ (a, Figs. 1 and 2), it 

 is necessary to recall the fact that it receives on either side, in the 

 arched folds of the mantle, the neck-portions of the valves of the shells. 

 By the contraction of the bundles of muscular fibres, the two valves 

 would separate slightly one from the other, a movement which is still 

 better understood if it is proved that that organ can become hard by 

 the afflux of the blood and thus furnish a better fulcrum for the action 

 of the muscles. Up to a certain point this part is similar to the hinge- 

 ligament of other bivalve mollusks; but only in this respect, that it 

 serves to open the shell. For the true ligament, wherever it exists, is 

 always composed of elastic tissue, and its action is purely passive, 

 while with the teredo the opening of the shell is a muscular action and 

 consequently active. Moreover, the hinge is wanting in this case, which 

 allows the supposition that the animal has the power of modifying at 

 will, by the partial contraction of its muscles, the direction in which 

 the valves separate, so that it may be at one time the middle parts and 

 at another the anterior parts of the valves which separate most from 

 each other. Besides, the effort which this action demands is extremely 

 feeble, and the movement of the valves themselves is very limited. 



There are two adductor muscles. The first and largest is already 

 well known ; it has been described by all writers who have made the 

 teredo an object of study. It extends (Fig. 11, m) between the two 

 valves in the form of a muscular mass, relatively quite large, and occu- 

 pies about two-thirds of the length of the shell and one-third of its 

 width. It rests on either side on a sort of pad situated at the limit 

 between the middle and neck parts of the valve. The second or small 

 adductor muscle, which appears to have escaped the attention of most 

 observers, is found near the dorsal side of the shell in the cavity be- 

 tween the anterior portions of the valves. One can see its exte- 

 rior surface, clothed with a thin epidermis and slightly projecting im- 

 mediately in front of the pallial prolongation, which extends over the 

 dorsal face of the shell ; in appearance it is only a continuation of this 

 muscle, but in reality it is entirely distinct. 



The principal mass of this muscle is implanted upon the sides, bent 



