402 Scientific Intelligence — Zoology. 



that three theories had been advocated as to the way in which mol- 

 luscs effect their entrance into the rocks, &c., in which they are 

 found. The first is, that the animal works with the shell in the 

 manner of a rasp or an augur. The second, that it secretes an acid, 

 whereby the substance with which it comes in contact is dissolved. 

 The third, that the effect is produced by the vibratile action of the 

 parts exciting constant currents of water against the substance, aided 

 by its impetus when drawn in down the elongated body of the ani- 

 mal. The author objected to all these theories. In opposition to 

 the first, he stated that the burrows are tortuous, and of a form to 

 render it impossible that the valves of the shell should act as a 

 centre-bit or augur. Again, the spines which cover many of the 

 shells are covered with an epidermis, which, if the shells were used 

 for boring, would be rubbed oflp, which is not the case. The young 

 of these creatures commence boring directly they are hatched, when 

 their shells are too delicate to produce any effect on the substances 

 into which they penetrate. In opposition to the theory that a sol- 

 vent is employed, the author stated that this must be of an acid nature; 

 and that by the most careful experiments, he had been unable to de- 

 tect the slightest indications of any acid secretion from any part of 

 the bodies of these animals. Besides, if a solvent were secreted, ca- 

 pable of dissolving calcareous and sandstone rocks, ought it not also to 

 act on the shell of the animal ? which was found not to be the case. 

 The physical appearances of the excavation, also, were not like those 

 produced by a solvent. It would not account, either, for their bor- 

 ing in wood. The wood found in the stomach of the Teredo navalis 

 was found to be chemically unchanged. The theory of the ciliary 

 currents was objected to, on the ground that it was inadequate to 

 explain the phenomena. If these delicate ciliary currents were ca- 

 pable of effecting this object, the dashing of the water over the same 

 surfaces ought to have as much more effect, as their physical force 

 is greater. The anatomical structure of the part, also, on which the 

 cilia are situated, would forbid the supposition that currents produced 

 by them could effect this object. These theories being insufficient 

 <o explain the phenomenon, the author proposed a new one. He 

 believes that the anterior portion of the animal is the excavating 

 instrument. This, in Teredo and Pholas, is composed of the foot 

 and edges of the mantle, which together fill up the frontal gape of 

 the shell. In Salicava and Gastrochoena, it is formed wholly of the 

 edges of the mantle, which are united and thickened. The form of 

 the excavation corresponds to the form of these organs. On a minute 

 examination of the surface of the foot of Teredo Norwegica, it is found, 

 underthe microscope,tcbecrowded with minute, brilliantpoints, which, 

 on being compressed, consist of comparatively large crystalline bodies 

 imbedded within them. These crystals are numerous, and of va- 

 rious sizes and shapes, chiefly five and six sided, but not by any 

 means regularly so. They all agree in having one or more elevated 



