Pholas.] 



MUSEUM OF ANIMATED NATURE. 



283 



mollusks live on extensive sandy beaches, or at the 

 mouths of rivers where a deep bed of silt affords 

 them the facility of burrowing; they bury them- 

 selves in a vertical position, with the foot down- 

 wards and the siphons upwards, the apertures of the 

 latter just projecting above the surface of the bed. 

 Here they quietly remain, taking in food and water 

 through the larger siphonic tube as the tide flows 

 over them ; when disturbed, down they plunge into 

 the oozy sand, disappearing with astonishing celerity, 

 often baffling the most active endeavours to capture 

 them, and making their way to the depth of several 

 feet. When the danger is past, they gradually re- 

 ascend, by the extension and contraction of the 

 powerful foot, but are ready in an instant to disap- 

 pear. The whole of their active existence is thus 

 passed in descending to the depths of their burrows 

 and in re-ascending to the surface. 



The foot is cylindrical, but alters its shape accord- 

 ing to need in the process of burrowing. 



The inhabitants of the coasts on which the Solens 

 are found, search for them, sometimes as food ibr 

 the poorest of the community, but generally as baits 

 for catching fish. The most favourable time for 

 taking them is after high tides ; they then often 

 appear just emerging from their burrows in great 

 numbers. The ordinary plan is to throw into their 

 burrows as they withdraw themselves a small quan- 

 tity of salt, which so irritates the animals that they 

 immediately ascend out of their holes, in order to 

 get rid of it. They are then seized, but some ad- 

 dress is required lest the animal should re-enter as 

 rapidly as it came forth. Another plan of taking 

 these shell-fish is by means of a long iron hook, 

 which the fisherman plunges deeply into the sand, 

 and drawing it out obliquely with a jerk, carries 

 away sand and solen also. 



Family PHOLAD.E (PHOLAS, &c.). 



The family Pholailae comprises a group of mol- 

 lusks, the boiing habits of which have been long 

 known ; they penetrate wood, hard clay, chalk, and 

 rocks, and devastate the labours of man ; they 

 attack the hulls of ships, submarine piles, the 

 foundations of piers and breakwaters, and conse- 

 quently become objects of anxious interest: they 

 force themselves u|)0n our attention by their in- 

 sidious but extensive depredations, the results of 

 which not only involve the loss of property, but often 

 also of life. 



The first genus to which we shall direct attention 

 is that termed Pholas, including its subgenera, 

 Xylophaga, Pholidaea, &c. 



In the genus Pholas the shell is delicate, white, 

 lather transparent, sometimes covered with a thin 

 epidermis, of an oval elongated figure, and gaping 

 at both extremities ; the umbones are hidden by a 

 callosity ; the hinge is toothless and without a true 

 ligament ; a flat recurved spoon-shape process, 

 enlarged at its extremity, elevates itself within each 

 valve below the umbo. The muscular impressions 

 are very distant, the posterior one is large and dis- 

 tinct, the anterior small, and often scarcely appa- 

 rent; the pallial mark is deeply furrowed posteriorly. 

 To the shell thus characterized are olten added 

 Cfilain accessory pieces ; sometimes a calcareous 

 tube envelopes all the parts, leaving an aperture pos- 

 teriorly. 



The mollusk, thus protected, is generally 

 elongated, with the mantle reflected on the dorsal 

 part, for the purpose of tying togetherthe valves and 

 the accessory pieces. The foot is short, oblong, and 

 flattened. The siphons are elongated and united 

 into a single very extensible and dilatable tube. 

 The mouth is small, with trifling labial appendages. 

 The branchiae are elongated and narrow, and pro- 

 longed into the inferior siphon. 



The number and disposition of the accessory 

 pieces in the shell of Pholas vary considerably ; and 

 it would appear that the animal, as it bores its way, 

 sometimes deposits a calcareous tubular linin;^ on 

 the internal wall of the cavity it inhabits. M. Rang 

 states that this fact he has not been able to verify 

 from living species, but that M. des Moulins showed 

 him several fossils from Mcrignac, in which he com- 

 pletely recognised this important character. 



A more clear idea of the forms of the animal and 

 shell of Pholas will be conceived by referring to our 

 pictorial specimens. 



Fig .3004 represents the animal and shell of Pholas 

 Dactylus ; the lower or ventral part is presented to 

 the spectator, showing the gape of the valves. 

 a is the mantle, open anteriorly for the protrusion of 

 the foot: 6, the foot ; c, the double siphon, emerging 

 from llie posterior aperture of the shell, d d. 



Fie. 3005 represents another species of Pholas 

 (Ph. clavatus ?), seen at a side view : a, the siphon ; 



b, the mantle ; c, the foot. 



Fig. 3(X)G represents the shell of Pholas Dactylus, 

 wi!h its accessory valves visible. A, the accessory 

 Vnlves: a, the anterior pair; fc, the central piece ; 



c, the posterior piece. B is an exterior lateral 



Vol.. il. 



view of the shell, with the accessory valves in situ. 

 C is an internal view of one of the valves : a, the 

 internal spoon-shaped process, under the umbo. 



Mr. Sowerby, in reference to Pholas, says, " We 

 believe that all the shells of this genus are furnished 

 with a greater or less number of accessory valves, 

 which appear to be caused by the deposition of 

 shelly matter (within the epidermis and connected 

 with the valves by that membrane), wherever such 

 valves were necessary for the security of the in- 

 mate. 



" They are consequently very various in form, and 

 placed in different situations in the different species, 

 though in most cases they are placed near the 

 hinge, and have even been considered to be substi- 

 tutes, in these shells, for the premanent ligament 

 of other bivalves. We must for the present with- 

 hold our assent from this opinion, because, on 

 account of the situation in which they live, the 

 animals inhabiting these shells can have very little 

 occasion to open their valves : whether or not there 

 is any premanent ligament in this genus, as we have 

 never observed the animal alive, 'we cannot under- 

 take to determine. Turton says it has none ; 

 Lamarck, on the contrary, speaks of the accessory 

 valves covering and hiding the ligament. As far 

 as we can form an opinion from dried specimens, 

 we cannot consider tlie substance to which these 

 valves are attached as the ligament, but as part of 

 the adductor muscle ; nevertheless we think we can 

 in some species perceive a very small internal liga- 

 ment, attached to two unequally-sized small curved 

 teeth, one in each valve, placed in the same situa- 

 tion as the hinge-teeth of common bivalves." 

 He adds, " The principal differences between Pholas 

 and Teredo consist in the latter forming a shelly 

 tube behind its valves, and in its being destitute of 

 accessory valves ; moreover the two valves of the 

 latter, when closed, are nearly globular." 



The same conchologist makes the following va- 

 luable remarks, in an introduction to the description 

 of eleven new species brought by Mr. Cuming 

 chiefly from the western parts of South America 

 and the islands of the South Pacific Ocean : — " The 

 utmost caution (he says) is necessary in the exami- 

 nation and description of the various sorts of Pho- 

 lades, on account of the extraordinaiy difference in 

 the form of the same species, in different stages of 

 growth. The addition of accessory valves, also, as 

 they increase in age, must be carefully observed, 

 in order to guard against too implicit a confidence 

 in their number and form. And though I might be 

 considered guilty of asserting a truism by stating 

 that the difference in size of different individuals of 

 the same species may and sometimes does mislead 

 the tyro in the science of Malacology, lest such 

 difference should mislead the adept also, let him, 

 too, proceed cautiously ; and when he finds a full- 

 grown shell of half an inch in length, agreeing per- 

 fectly in proportions and characters with another 

 of two inches long, let him not conclude that it is 

 a distinct species, but if he can find no other dif- 

 ference except that which exists in their dimensions, 

 let him consider the one a giant, the other a dwarf. 

 Let it be remembered that among the Cypra^ae it is 

 not uncommon to observe young shells of three 

 inches in length, and fully grown ones of the same 

 sort only an inch in length. Likewise of the 

 British Pholades there are individuals quite in a 

 young state of two inches in length, and perfectly 

 formed shells of the same species not more than 

 half an inch long. For instance, in demonstration 

 I need only refer to the Pholas papyraceus, so 

 abundant at Torquay, of which the young shells 

 have been considered by many as a distinct species, 

 and have been named by Dr. Turton Ph. lamellosus. 

 This varies in size exceedingly, so that it may be 

 obtained both in an incomplete and young stale, 

 and in a fully grown condition, from half an inch to 

 nearly two inches in length. The circumstance of 

 its having rarely occurred in an intermediate state 

 of growth, when the anterior opening is only partly 

 closed and the accessory valves only partly formed, 

 led Dr. Turton and others to persist in regarding the 

 young and old as two distinct species." 



M. Deshayes says that there is no true ligament 

 in Pholas, but that a part of the anterior muscle is 

 inserted on the cardinal callosities, and occupies the 

 place of a ligament. In Teredo, also, there is 

 no true ligament. With respect to the internal 

 spoon-shaped processes, they are buried in the 

 thickness of the animal, and embrace in their 

 concavity a part of the liver, the lieart, and in- 

 testines. 



The genus Pholas is very widely distributed ; and 

 all the species manifest the same boring habits as 

 those in our own and the adjacent coasts, and of the 

 ravages of which the Breakwater at Plymouth 

 affords proof. We have now before us a piece of 

 hard chalk completely mined in all directions with 

 pholades, and which we picked up near Ramsgate, 

 where other masses of the same character were 

 seen in abundance, and filled with the empty and 



dead shells of the borers. Some of Mr. Cuming's 

 specimens were in soft stone, others in haid stones, 

 others again in limestone, hard clay, decayed wood, 

 and the trunks of trees at low water. With respect 

 to the means by which these creatures effect their bor- 

 ing operations, many opinions have been entertained. 

 Some have attributed the perforations to a rota- 

 tory motion of the shell, by which the stone or chalk 

 is, as it were, rasped away. But as the shells fit 

 the cavities in which they are lodged, this opinion 

 hasnosupport ; besides, one would think that the ex- 

 tremely delicate valves of Pholas would themselves 

 become worn down by such a process sooner than 

 rock. Others, again, have attributed it to the action 

 of currents of water produced by the vibratile cilia of 

 the animal, and directed so as to act anteriorly to the 

 animal, which presses onwards as the currents wear 

 down the stone. It is possible to conceive that 

 such currents may take an effect on soft materials 

 of which the particles become readily disintegrated, 

 but when we see solid blocks "of timber, as oak, 

 riddled in every direction, we cannot but hesitate 

 as to the part which ciliary currents take in such 

 perforations. The constituent particles of oak are, 

 one would think, too adherent to yield to minute 

 ciliary currents of water, the force of which must be 

 very trifling. Others, again, have attributed the whole 

 to the agency of some chemical solvent poured out 

 by the mollusk. To say nothing of the danger to 

 which its own shell would be exposed, this theory is 

 discountenanced by the circumstance that the rocks 

 or stones bored are of dift'erent natures, as limestone, 

 clays, sandstones, &c., to say nothing of wood. Now 

 we can scarcely suppose that the same chemical 

 agent will dissolve one and all of these substances ; 

 unless, indeed, we are to suppose that each species 

 bores only into one given material, a circumstance 

 which we do not know there is any ground for sup- 

 posing, though perhaps some, as the delicate Pholas 

 conoides, may be oftener found in hard wood than 

 in stone. 



Mr. Sowerby, in his notice of the Pholas acumi- 

 nata (loco supra diet.), found at Panama in argilla- 

 ceous limestone, at low water, says, "This species 

 demonstrates a fact of considerable importance to 

 geologists. It is in argillaceous limestone, very 

 much resembling lias, and in forming the cavity 

 in which it resides it has, by such a chemical process 

 as frequently takes place, absorbed a much greater 

 quantity of the rock than could be retained or con- 

 verted. This is again deposited at the upper part of 

 the cavity, and thus the rock is recomposed." We 

 think this fact equally as valuable to the zoologist 

 as to the geologist. It seems to indicate that it is 

 by absorption that the tunnel is bored. May not 

 the broad foot, we would ask, be a powerful organ 

 of absorption, and be in constant application, like a 

 sucker to the end of the tunnel, throwing into the 

 system the matter taken up, and which is again 

 thrown out through the upper of the two siphons ; 

 or, as in the instance of Pholas acuminata, re- 

 deposited at the upper part of the cavity so as to 

 reconipose the rock as the mollusk pushes onwards ; 

 And further, may not the tube lining the internal 

 wall of the cavity, as observed by M. des Moulins 

 in the instance of the fossil Pholades from Mfirignac, 

 have been produced by the excreted materials pre- 

 viously absorbed ? According to this view, the 

 foot would serve a double purpose : adherent to 

 the extremity of the tunnel, it would carry on the 

 excavation by absorption, and draw the animal 

 forwards, in accordance with the rapidity of the pro- 

 cess. 



Let it not be supposed that we wish to be positive 

 on a subject which, alter all, is involved in much 

 obscurity. 



A better idea than mere words will convey of 

 the destructive labours of the Pholas, may be con- 

 ceived by reference to our pictorial specimens. 



Fig. 3007 represents the Pholas striatus in wood : 

 the mass is completely riddled by the animals, to 

 the shape of which it will be seen the cavities are 

 precisely fitted. 



Fig. 3008 shows a block of stone perforated by the 

 Pholas Dactylus. 



Fig. 3009 is a block of wood perforated by Pholas 

 dorsalis (Xylophaga dorsalis). 



Fig. 3010 represents the shell of Pholas dorsalis, 

 the type of the subgenus Xylophaga. a, an enlarged 

 view of the interior of the valves ; b, a doi-sal 

 view, natural size ; c, a ventral view, natural size. 



Fig. 3011 represents the Pholas jiapyraceus, com- 

 mon on the coast of Devonshire, belonging to the 

 genus or subgenus Pholidaea of Leach. Ttie shell, 

 with the animal inclosed, is seen in a side view at a ; 

 in a dorsal view at 6 ; c, a terminal cup-shaped 

 membrane. 



The fossil species of Pholas are rare ; they occur 

 in strata below the chalk, and also in tertiary for- 

 mations. 



Within the limits of the Family Pho'alne we place 

 the genera Teredina and Teredo, part of the Tabu- 

 lida; of Lamarck. 



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