4*7 



LITUOI'UAGID^E. 



LITHUPHAGIDJi. 



required to enlarge iti habitation the adductors perform a very 

 essential part The animal being attached by the foot brings the 

 anterior part of the shell into contact. The lateral muscles now 

 contract, and raiting the posterior adductor bring the dorsal margins 

 of the valve* into contact, so that the strong rasp-like portions are 

 suddenly separated, and scrape rapidly and forcibly over the substance 

 on which they press. As soon as this is effected the posterior extre- 

 mity sinks, and the stroke is immediately repeated by the successive 

 contraction of the anterior adductor, the lateral, and the posterior 

 adductor muscles. 



" ' The particles rubbed off, and which in a short time completely 

 clog the shell, are removed in a very simple manner. When the 

 projected syphon is distended with water, the Pholat closes the 

 orifices of the tubes, and retracts them suddenly. The water which 

 they contained is thus ejected forcibly from the opening in the mantle, 

 and the jet is prolonged by the gradual closure of the valves to expel 

 the water contained within the shells. The chamber occupied by the 

 animal is thus completely cleansed; but as many of the particles 

 washed out of it will be deposited before they reach the mouth of 

 the hole, the passage along which the Pholat projects its syphon is 

 constantly found to be lined with a soft mud.' 



" In a valuable paper' On the Anatomy of the Lamellibronchiate Mol- 

 luscs,' published by Mr. Garner, in the 2nd vol. of the ' Zoological Trans- 

 actions,' that excellent observer has some short remarks on the means 

 by which Pholat and other molluscs bore. He mentions that the boring 

 is effected by currents produced by vibratile cilia, aided in some cases 

 by rasping. He remarks that the valves of Lithodomut are not adapted 

 for mechanical boring; that the crypts of Saxicava are not circular; 

 that the valves of Teredo, probably do not correspond to the bore ; 

 that Pholat conovlti is often found in hard timber, though its valves 

 are not adapted for any boring or filing. He further observes, ' There 

 is a cartilage between the two small spinous processes of the binge 

 in the Pholat amdidui ; in other species of Pholat which have no rudi- 

 ment of it, and allied genera which have a particular character of arti- 

 culation, I consider the motion of the valves as but a secondary cause 

 in the perforation of the substances in which the animals are found.' 



" Mr. O. It Sowerby objected to the notion that the cavities of 

 Pkolat were produced by rotatory motions of the shell, since they 

 are fitted to the latter, and since some of the shell bones are externally 

 xmeoth. He also objected to the notion of a solvent. Mr. W. 

 Thompson, in his paper ' On Teredo,' expresses his belief that the 

 smoothly-rounded termination of the cell made by that animal is due 

 to the action of a solvent supplied by the proboscis, which thus acts 

 as a pioneer in mining the passage that is afterwards increased to 

 its final dimensions by the boring action of the primary valves." 



I >r. Drummond, in his ' Letters to a Young Naturalist,' suggested 

 the possibility of the animal of the boring molluscs decomposing the 

 ana-salt as its wants may require, and applying the liberated muriatic 

 acid to the solution of the calcareous rocks. 



X. Deahayes, in his work ' On the Mollusca of Algeria,' maintains 

 that the Teredo bores by means of a solvent The foot be regards as 

 a special secreting organ. M. F. Caillaud holds a similar opinion with 

 respect to the perforating power of animals of the genus Claragclla. 

 In the Anunles des Sciences Naturelles' for 1839, M. L. A. Necker 

 published some very interesting and important observations on the 

 mineralogical nature of the shells of Mollutca, in which he points out 

 difference* of structure as indicated by polarisation of light In it 

 he has the following observations on the subject in question : " It is 

 very remarkable that two genera of perforating shells, the Pholat and 

 the raterupii, radiate strongly calcspar. Thus the asperities which 

 roughen their shells may, with the aid of the acids with which they 

 are provided, excavate the calcareous rocks inhabited by them. The 

 notion that their shells are formed of calcspar seems to render impos- 

 aible the perforation by the shells themselves of calcareous rocks 

 equally bard with the bodies perforating. But now that we know 

 these shells are composed of amgonite, it is plain that they can act 

 mechanically even on the hardest limestones." 



Sir Henry da la Beche has given a table of the specific gravity of 

 ome shells as observed by himself, in which he states that of Pholat 

 cfitptla to be 2*2, and of a number of other bivalves and univalves to 

 be nearly the same ; indicating a constitution which, allowance being 

 made for amount of animal matter, approximates their shells much 

 more nearly to amgonite than to calcspar, the former having a specific 

 gravity of -JUS and the Utter of 2-71. 



Mr. Albany Hancock has put forward one of the most definite 

 and important opinions ever expressed on this disputed question. 

 He sUtes that "the excavating instrument of Pholat and Teredo 

 is formed of the anterior portion of the animal, in the surface of 

 which are embedded siliceous particles. The particles penetrating 

 the skin give to it much the character of rasping-paper. The whole 

 forms a nibbing surface, which, being applied closely to* the bottom 

 of the cavity l,y the adhesion of the foot, enables* the animal to rub 

 down ami no penetrate shale, chalk, wood, or even the hardest 

 limestones and marble. Saricara rtu/ona is also furninhed with 

 a rasping surface covered with siliceous particles. This surface 

 however in this specirs is formed entirely of the anterior portion of 

 the mantle, the margins of which being united are so much thickened, 

 forming sort of cushion capable of considerable protrusion at the 



will of the animal. The foot is small, and, passing through a much- 

 constricted orifice, gives origin to a byssus, which anchors the shell 

 close to the base of the excavation, and thus holds the rubbing 

 apparatus in immediate contact with the part to be excavated." 



In summing up all these theories, Messrs. Forbes and Hauley give it 

 as their opinion that " the chemical one, so far as a secreted solvent is 

 concerned, bears the least examination in the case of the Pkoladida, 

 The substances perforated are wood, limestones hard and soft, argilla- 

 ceous shales, clays, sandstone, and, in the case of a Pltolca iu the magni- 

 ficent collection of Mr. Cuming, wax. The notion of a secreted solvent 

 that would act indifferently on all these substances is, at present at least, 

 purely hypothetical, and since all attempted tests hare failed to detect 

 an acid, gratuitously so ; for we can hardly suppose that any of those 

 who have taken this view of the cause would maintain that the animals 

 have the power of secreting different acids at will, according to the 

 substance they have to attack. Vet this notion has been most favoured 

 by naturalists who, sceptical as to the perforating power of such fragile 

 instruments as are the shells of many of these creatures, endowed the 

 animals with supernatural chemical qualifications. Even good expe- 

 rimental observers, Mr. Osier for one, whilst they proved that the 

 Pholat could bore mechanically by the rotation of its valves, could 

 not free their minds from the prejudice in favour of a solvent. The 

 important statement put forward by Mr. Albany Hancock respecting 

 the instruments by which Molliuca bore, and which, BO far as Gattero- 

 poda are concerned, appear to furnish us with a true explanation, 

 namely, that it was effected by means of siliceous particles variously 

 arranged in certain portions of the animal's body, led us to hope that 

 a better cause than any yet alleged had been discovered. But we 

 cannot bear it out with respect to the Pholadidic. We can find no 

 such particles in the mantle of Teredo, nor have any been noticed by 

 Home or Deshayes, or by the most recent observers, Frey and 

 Leuckart, who paid especial attention to the structure of the tissues 

 of this genus. Nor could we, though aided by the anatomical and 

 microscopical skill of Mr. Buak, detect any siliceous particles in 

 either the mantle, foot, or siphon-tube of Pliolas candidui. If present 

 in any species therefore they are exceptional, so far as the genus 

 Pholat and its allies are concerned. The shells of several British 

 species of Pkolat and that of Plioladulix have been chemically 

 examined by our friend Mr. Trenham Reekes with a negative result as 

 regards the presence of particles of silex in their substance, where, 

 after the statement of Mr. Hancock respecting the structure of the 

 mantle, we thought they might possibly be found. On the other hand, 

 taking into consideration its mineralogical nature as stated by M. 

 Necker, there is no reason for supposing that the shell of the Phola- 

 diila is so weak a perforating instrument as some have fancied. With 

 its peculiar form and the saw-like asperities of its surface, especially 

 of iU autial extremity, it is well adapted for an auger when wielded 

 fresh and elastic by iu well-muscled inhabitant, whose foot in all the 

 members of this tribe, even in Teredo, where it is least developed, 

 seems especially organised to serve as a fulcrum. We have no evidence 

 that they perforate any substances essentially harder than their shell*, 

 or BO hard. The sandstones iu which they occasionally occur are 

 either friable or marly when fresh, though cabinet-specimens seem so 

 solid. The explanation of Necker accounts for their perforations in 

 the hardest limestones. Wood, wax, and other substances in which 

 they occur, offer no difficulty. The statements put forward respecting 

 their boring in lava and granite have long ago been shown to be mis- 

 takes. That they exhibit a rotatory motion during the action of boring 

 has been proved by competent observers ; and the cavities they exca- 

 vate, if examined fresh, invariably show transverse groovings, which 

 could have been caused only by such motions. Currents of water set 

 in motion by cilia doubtless aid materially the animal's operations, 

 and possibly may be the means by which the larva) effect their first 

 lodgment ; but considering the arrangements of the parts of the body 

 in the adult animal, it seems to us that Mr. Garner's view of their 

 being the primary cause of the perforation, whilst the rasping of the 

 valves is secondary, should be reversed. Such currents must be most 

 effective in clearing away loosened and loosening particles. If there 

 be any chemical action aiding, it must be due to the carbonic acid set 

 free during the respiratory process. Evidences of a secreted solvent 

 there is none." 



\Vc proceed in this article to the examination of some of the 

 excavating Lamellibranchiate Conchifers which are most remarkable 

 for their boring powers. 



upit. This form in placed by Mr. Garner in that section of 

 the Dimyaria (with two adductor muscles) which is distinguished by 

 having the branchiae united medianly ; and the characteristic of Vent- 

 rupii, as given by the same author, is to have the tubes large, and the 

 foot short and prominent behind. 



Tic animal in oblong, rather thick, having the borders of the 

 mantle simple, slightly open before for the passage of a compressed 

 and elongated foot; tubes two in number, rather long, united in a 

 connidcrablc portion of their length, and having their orinces radiated; 

 branchhe little and unequal ; labial appendages very small. 



Shell solid, striated, or radiated, a little elongated, gaping posteriorly, 

 more or less irregular, equilateral, very inequivalve, the anterior side 

 being always shorter than the posterior side, which is generally trun- 

 cated as it were, the other being more or less rounded ; uniboned 



