CONCHIFERA.] 



MUSEUM OF ANIMATED NATURE. 



255 



double posteiior siphon, protruded from the mouth 

 of tlieir burrow, for the purpose of respiratiou and 

 the discharire of effete matter. 



Fiij. 28U5 represents the Cyclas Cornea : its large 

 foot protruded, and two posterior siphons are very 

 conspicuous. 



Fisr. 28G6 represents a common fresh-water she!!, 

 the Uiiio pictorum. A, the anterior extremity of 

 the shell ; P, the posterior extremity, with the two 

 siphons or tubes ; H, the hinge ; F, Ihe foot. 



We have hitherto only alluded to the mantle of 

 such Bivalves as when their shells are closed are 

 completely shut up, no aperture being lelt, the valves 

 tlierefore requiring to be opened for the protrusion 

 of the siphon. But there is a group called ' Les 

 Enfermes'by Cuvier, because the moIlusUsare com- 

 pletely enclosed in their nrsntie, wliile the valves 

 remain open, from their shape, at each extremity. 

 Such are the Soien, or Razor-stiell, the Pholas, and 

 others. The sliell of the Solen is long and cylindri- 

 cal, and open at both ends: the siphcn, or double 

 tube, is protruded from the posterior orifice, and the 

 foot from the anterior, near to which, on the back 

 edge of the valves, interiorly, are two or three 

 tooth-like projections, fitting into each other. The 

 rapidity with which the Solen or Razor-shell, called 

 by the French manche de couteau, or knife-handle, 

 can bury itself in the sand is very remarkable. Its 

 foot is the instrument employed for this purpose, 

 and the depth the creature penetrates is often several 

 feet, baffling all endeavours to capture it. 



In the rock-boring Pholas the two valves of the 

 shell are convex, broad anteriorly, becoming narrow 

 posteriorly. A large oblique fissure is left at each 

 extremity. Through the anterior fissure the foot is 

 protruded ; the double tube through the other. 



In the Teredo the mantle is produced into asiphon, 

 much longer than the two small rhomboidal valves ; 

 and this tube is always so placed, while the animal 

 is boring its way, as to have free access to the water. 



We have already said that it is ths mantle which 

 secretes the shell. The latter, as we know, consists 

 of carbonate of lime, cemented together by a viscid 

 animal matter, which dries, the whole becoming 

 hard and solid. 



Now, if we examine the mantle, say of a muscle, 

 ■vihieh we shall find lining the shells, and enwrapping 

 the body, we shall find its margins considerably 

 thickened, and adherent to the edge of each valve. 



In the oyster they are quite free, but still thick- 

 ened ; and this thickened edge is glandular, and 

 often, as in the scallop, furnished with a delicate 

 fringe of sensitive tentacles. 



In many instances it is seen to contain certain 

 patches of different colours, corresponding both in 

 tint and relative position with those presented upon 

 the surface of the shell. These coloured patches are 

 glandular, and owe their colouring to tlie pigment 

 they themselves secrete. 



Now, in order to increase the dimensions of the 

 shell, ifie margins of the mantle are protruded, each 

 being firmly adherent to the circumference of the 

 respective valve which it lines. The margin then 

 pours out the mixture of gluten and lime, depositing 

 it on the edge of the shell, when it hardens, and 

 remains fixed. At intervals this- process is repeated, 

 and every newly formed layer enlarges the extent of 

 the valve. The concentric strata, thus deposited, 

 remain distinguishable externally, marking the pro- 

 gress of increase by these lines of successive addi- 

 tions. 



At certain times the deposition of shelly matter 

 is more abundant than at others ; and the size and 

 distance of the ridges denote this circumstance. 

 Sometimes at such periodical epochs the mantle 

 shoots out, by a sudden development, beyond its 

 usual extent, depositing broad raised plates, or spines, 

 which remain permanent, the mantle retiring to its 

 ordinary state : another epoch comes, and a fresh 

 plate, or row of spines, is added, and so on, with 

 intervals, marking the successive developments of 

 the mantle, produced by the periodical stimulus to 

 increased action. 



While the shell-secreting glands pour out their 

 produce, the colour-glands pour out their pigment, 

 mixing it with the yet viscid matter composing the 

 shell, which it tinges. When this secretion of colour- 

 ing matter is kept up uninterruptedly, the tinted 

 marks or lines of the shell will be unbroken ; but if 

 the secretion be poured out at intervals, the mark- 

 ings will be broken, and spots, dashes, and other 

 ornamental stains will be the result; and these will 

 be larger and bolder as the shell increases in growth. 



We have said that a viscid secretion is mixed 

 with the carbonate of lime in shells: if, as is the 

 case in some species, this viscid fluid is in more than 

 sufficient abundance to set the fime, it forms on 

 the surface a sort of parchment-like tissue, or a 

 horny layer, generally known under the name of 

 Epidermis. In some species this layer is thick, — in 

 some it becomes loose, fibrous, and shaggy, like 

 coarse hairs, or the fibrous investment of a cocoa- 

 nut. 



Shells are lined, as we know, with a smooth, glossy, 

 and often iridescent coating, called mother of pearl, 

 or nacre. The deposit of this in sou.e species is 

 very abundant, giving great thickness and solidity 

 to the shell. This nacre is secreted by the external 

 surface of the mouth, not by its fringed and thickened 

 edge. It is, in fact, added as a smuoth coating after 

 the hardening of the shell, and constantly added to 

 as the valves increase. 



Pearls, we need scarcely repeat, are nothing but 

 nacre : they are the products of the mantle under 

 certain circumstances. Local irritation of various 

 kinds will cause their formation : minute grains of 

 sand, conveyed by accident between the valves, and 

 sticking to the mantle, often form the nuclei of 

 pearls. They become covered with concentric layers 

 of nacre, which soon increase, forming detached glob- 

 ular pearls. Many of the minute boring annelids 

 perforate shells, when the mantle, by way of block- 

 ing them out, secretes under the effects of the stimu- 

 lus an unusual and irregular abundance of nacre, 

 which, enlarged by successive layers, forms pearls 

 adherent by a wider or narrower base to the interior 

 of the valve. Linnaeus stated that he possessed the 

 art of procuring pearls at pleasure by drilling a 

 small hole in the shells, and then introducing through 

 it a grain of sand, or the like. 



We have stated that the valves of these mollusks 

 are closed by means of a muscle or muscles, which 

 counteract by their contraction the elastic spring of 

 the hinge. Now on a little consideration it will be 

 plain that, as the shell increases by the addition of 

 matter to the vential edge, the position of the 

 adductor muscle or muscles must gradually change 

 in order to maintain their relative place to the 

 circumference of the shell. They must in fact 

 maintain the same situation in the adult as in the 

 young. In the oyster, for example, it is quite ob- 

 vious that the adductor muscle, which was connected 

 with the thin and minute lamellfB forming the 

 first shell, has duiing its growth become farther re- 

 moved from the hinge, and transferred moreover 

 from layer to layer of nacre, as the shell increased 

 in thickness, and its circumference enlarged. 



The question arises, how does this muscle, which 

 we find so firmly adherent to the nacre lining the 

 valves, become thus altered in its position? It must 

 then first be premised that there is a thin layer 

 of the mantle interposed between the muscle and 

 the shell, so that in fact the muscle is not extra- 

 palliate ; and this thin part of the mantle increases 

 the nacre, which adds to the thickness of the shell, 

 as much under the extremities of the muscle, as the 

 rest of the surface of the mantle does elsewheie. 

 Here then we have a clue to the gradual shifting of 

 the muscle. Particle after paificle is laid on be- 

 tween the muscle and the shell, whilst the rest of the 

 mantle is adding equally to the general surface of 

 the valves, so that a new layer has formed, extend- 

 ing farther than the previous layer, to meet the in- 

 crease of the shell; and thus is the adhesion of the 

 muscle imperceplibly transferred to a new surface 

 of nacre, and at the same time advanced forwards; 

 and thus, during the growth of the shell and niol- 

 lusk, is this operation insensibly but gradually tak- 

 ing place : so that, if we were to peel off the nacre 

 in layers, we should find the muscle-mark in a more 

 and more backward position as layer after layer was 

 removed. In short, the growth of the shell and the 

 addition of a fresh and more extensive layer of nacre 

 are more or less consentaneous, and each increased 

 layer of the latter will carry the muscle forward, so 

 that it will always retain its due relative position. 

 The bivalve mollusks are most probably all bi- 

 sexual. The eggs in many are not abandoned at 

 the time of their exclusion, but are deposited be- 

 tween the membranes of the branchial laminae, ex- 

 posed to the ciliary currents of water, where they 

 undergo a sort of incubation. In some in fact the 

 shell is developed before they quit this receptacle. 

 Sir Anthony Carlisle says, "Oysters are viviparous, 

 and the young are found within the tracheal (bran- 

 chial) passages, and between the folds of the cover- 

 let (mantle) during the months of June and July in 

 this climate. In its first state the oyster exhibits 

 two semi-orbicular films of transparent shell which 

 are continually opening and closing at intervals. 

 The whole brood are associated together by being 

 involved in a viscid slime, and in that state called 

 spat, it being common among viviparous animals of 

 this kind to have their spawn posited in contact with 

 the lungs (branchiae) ; the involving slime serves as 

 the first nwtriment, and we may infer that the food 

 so influenced by the gills (branchiae) is at the same 

 time a respiratory supply to the imperfectly formed 

 young." 



The breeding months of the oyster are May, June, 

 and July, and during this season they are unfit for 

 food. When the sp(U is ripe for being deposited, it 

 becomes attached to stones, rocks, shells, &c., and 

 myriads of young become developed at the same 

 time, forming immense beds, for the supply of man. 



It is, however, from artificial rather than from 



natural beds that the vast quantities are obtained 

 which supply our markets. Biood or young oysters 

 of small size are collected and transplanted into 

 favourable spots, which are strictly protected, and 

 become a productive source of profit both to the 

 dredgers and the public. In parts of the coast 

 where no particular rights exist, the beds become 

 much thinned, and the brood is often carried away 

 and planted in beds under protection, where they 

 multiply in abundance. Newly formed beds are 

 generally kept untouched for two or three years, to 

 allow for the growth of the young. Of the age at- 

 tained by the oyster little is known ; some suppose 

 it to be about ten years. In three days after the 

 spawn is deposited, the young oysters are enclosed 

 in a shell three lines broad ; in six months the shell 

 is nearly as large as a half-crown piece ; at the end 

 of a year it equals a dollar. After a certain period 

 the mollusk ceases to grow, the shell is large 

 in proportion to the bulk of tbe animal, which 

 becomes thin, gradually diminishes in size, and is 

 unfit for food. Oysters, when put to fatten into 

 small pits in the saline marshes, as we have seen 

 along the Swale, with the water about three feet 

 deep and abounding with marine vegetable matter 

 and animalcules, assume a green tinge in three or 

 four days, and are excellent. The oysters taken at 

 Dieppe are of a greenish hue. In Spain they have 

 a reddish tinge. 



The most celebrated beds for native oysters are 

 those at Milton in Kent, at Colchester, Maldon, 

 Feversham, and Queensborough. Those in the 

 Swale and Medway are in high repute. In Scot- 

 land the beds in the Frith of Forth and in Mussel- 

 burgh Bay afford good oysters. In France the 

 oysters from Brittany have been long famous ; those 

 from Dieppe are very excellent, but the oysters ob- 

 tained near Cancalle, a town not far from St. Malo, 

 have the highest reputation. From the Swale and 

 from Rochester and Colchester vast numbers of 

 oysters are exported to Holland and Germany. 

 From the island of Jersey 200,000 bushels are annu- 

 ally exported, and 2rj0 boats, 1500 men, and 1000 

 women and children are employed in the season. 

 Dredging for oysters is carried on generally in fleets, 

 as the beds lie within a comparatively small space. 

 The boats are about fifteen feet long, and usually 

 carry two men and a boy. The dredge is about 

 eighteen pounds' weight, 'but it is requiied to be 

 heavier on a hard than a soft bottom-. Each boat; 

 is provided with two dredges ; but the fishermea 

 complain that in the early part of the season too 

 great a number of dredges, and those of too heavy a 

 kind, are used, which injure the beds, so that the 

 latter part of the season is rendered less profitable 

 than tFie commencement. 



A fleet of oyster-boats putting out early in the 

 morning, crossing and intersecting each other's 

 course as they advance to the fishing-ground, their 

 white sails glancing in the sun, and anon as the 

 boats tack about becoming shadowed, and again 

 brightly glancing, is a most interesting spectacle. 

 Such a scene is represented at Fig. 28G7. Fig. 

 28C8 represents an oyster-dredger in his usual dress, 

 casting the dredge over the gunwale of his boat. 

 I From this introduction we may advance to the 

 consideration of our pictorial specimens, of which 

 , we first select those belonging to the scallop and 

 oyster family. 



MONOMYARIA. 

 Family PECTINID^ (Scallops, Oysters). 

 The shells of bivalves have what is termed a right 

 and a left valve ; and which is the right and whict* 

 the left valve may be generally determined by 

 placing the shell on its edge with the anterior part, 

 denoted by the umbo and lunule, forwards, and the 

 hinge and ligament next the observer.* We notice 

 this because M. de Blainville divides the Pectens ■ 

 into four groups : first, into those specimens which 

 are very inequivalve, the left valve being very flat, 

 as in the common scallop; secondly, into equivalve 

 species ; thirdly, into species in which the two valves 

 are nearly equally concave, hut the right rather the 

 least; and, fourthly, into such as have striae paialiel 

 to their border. Mr. Sovverby makes five divisions. 

 Now in the Pecten, from the situation of the umbo 

 and the characters and place of the hinge, the right 

 and left sides can only be determined by the position 

 of the animal. 



The genus Pecten is thus characterized by M. 

 Deshayes. Mollusk subrotund, not thick ; lobes of 

 the mantle very delicate, disunited throughout, 

 thickened on the borders, and furnished with many 

 rows of fleshy cilia, between which are regularly 



* Many reverse this, and call the liinule portion of the stiell th« 

 posterior, and the !on{,' p:irt, with tlie hinye, and with the siphons o* 

 the mollusk, the anterior part ; indeed, Mr. G. IJ. Sowerby states ttiis 

 to lie the general plan : if so, it is contrary to common sense ; much 

 confusion and contradiction, however, accrue from it, as it reverses 

 tlie valves, and makes wliat oii^ht to be called tlie left the right, and 

 vice ven,a. In our views it is the right valve of the oyster that is the 

 convex one, the left tile flat one. " II I'aul se souvenir (dit Cuvier) 

 que lo ligament fde la chaniijre) eJt toujours du cote postcrieur de» 

 sommets' tumbones). 



