326 



THE OCEAN WORLD. 



it suffices to explain the excavation of the galleries, however extensive 

 their ramifications. Again, the upper cutaneous folds, especially the 

 cephalic hood, having the power of expanding at will by an afflux of 

 blood, being covered with a thick coriaceous epidermis, and moved 

 by four strong muscles, seems to me very capable of performing the 

 operation. It appears very probable that it is this hood which is 

 charged with the removal of the woody fibre, rendering it incapable 

 of resistance by previous maceration, which may also be assisted by 

 some secretion from the animal." That the fleshy parts of the mollusc, 



acting upon the surface, softened 

 by long maceration in water, is 

 the true boring implement em- 

 ployed by the Teredo, is, pro- 

 bably, the only explanation the 

 case admits of; at all events, in 

 the present state of our know- 

 ledge, the explanation of this 

 naturalist is the most reasonable 

 \vhich can be given. 



The engraving (Fig. 129) re- 

 presents Pholas dadyhis, which 

 has hollowed itself a home out of 

 a block of gneiss. This dwelling 

 is a cell just deep enough to con- 

 tain the animal and its shell 

 (Fig. 130). To excavate its cell 

 at the bottom of one of these 

 gloomy retreats seems to be all 

 that the animal lives for. To 

 ascend to the summit or sink to 

 the bottom of their narrow house 

 makes up all the accidents of existence to these strange creatures : 

 the hole they dig is at once their dwelling and their grave ; which 

 fact is attested both by the rocks of the past and the present. 



In its structure the shell of this genus differs notably from other 

 Acephalous IMolluscs, which led Linnaeus to place it in a section 

 which he made of multivalve shells. Between the two ordinary 

 valves, in short, this shell presents certain accessory- pieces, smaller 

 than the true valves, and placed near the hinge, as represented in 

 Pholas dadylus (Fig. 129), pieces which would not be there without 

 a purpose. 



The shell is equivalve, gaping on each side, swelling below, very 



Fig. 130. — Pholas dactylus (Linna;us). 



