251 



MUSEUM OF ANIMATED NATURE. 



[CoNCUIFEnA. 



vier: LamelUbranchiata, De Blaiiiville). 

 thU claiw Lamaick aiisociatei all the 



Under 

 molluiks 



inclosed in bivalve klielU, inc'udin!^ also the 

 Biachiupoda, which however constilute in the 

 arrani;enierit of Ctivier a distinct c1hi>s, and as such 

 we shall consider them. 



The number of species belont^ing to tlie Conchi- 

 fera almost exceeds imagination. They pave the 

 shallower parts of the ocean, where the shells, ac- 

 cunuilatini; as the animals die, form deep beds, 

 which at some future ejioch, when the sea shall have 

 left lis present situation, may attract the attention 

 Of ffeolosists. and excite their' speculations as to the 

 condition of this planet at the remote epoch of their 

 dejiosilion. It is thus, as we examine the various 

 strata, the clays, the chalks, tlie limestone rocks 

 wliich abound in the fossil bivalves of ages past, 

 that we muse over the state of our globe when the 

 sea was where hills and rocks are elevated ; and 

 thus shall it be again, as Time rolls on, developing 

 the ceaseless agencies of change which lie hid in this 

 planet, or are inseparable from the operations, che- 

 mical or mechanical, of Nature hersell'. 



The Conihil'era are invariably aquatic, and, as we 

 see in the oyster, are attached to the valves by one 

 or more muscles destined for closing the valves toge- 

 ther, and retaining them firmly shut. Such shells as 

 have only one adductor muscle are termed Mono- 

 mi/arinns; such as have two are termed X)(/«yan'a;w. 

 In The Monomyarian shells it is the posterior muscle 

 which is present, and tliere are often some very mi- 

 nute traces of an anterior muscle. The valves are 

 united at their back by means of a hinge ; this hinge 

 is formed by the inner layer of the shell, and con- 

 sists either of a simple cardinal process, or of a ser- 

 rated edge, or of projections or teeth which fit into 

 corresponding cavities. To this is superadded a 

 ligament which binds the two parts together, and 

 keepsihe teeth or projections in their places. This 

 ligament is either internal (being hidden by the car- 

 dinal edge) or external. It is wonderfully elastic, 

 composed of fibres compacted together, and per- 

 pendicular to the valves they connect; and its office 

 IS to open the shell when the adductor muscles, of 

 which it is the antagonist, are relaxed. After the 

 death of the animal, when these muscles lose their 

 power, owing to this elasticity of the ligament the 

 valves gape wide, more so than during the life of 

 the mollusk. 



But before proceeding, we may here give a brief 

 explanation or the external characters of a bivalve 

 shell, and for this purpose let us consult the Figs. 

 2859. 28G0, and 2861. The form varies,— that of 

 Cytheraea we select as an example. The semi- 

 circular lines on the upper surface at Fig. 2s59 indi- 

 cate the stages of progressive increase, by deposition 

 from the mantle or pallium, which, as we shall here- 

 after explain, covers the animal. The line from a 

 to p gives the longitudinal measurement of the 

 shell ; the cross line indicates its height. The 

 rounded and more or less elevated apical portion is 

 termed the Umbo, and anteriorly but below this is 

 a depressed space, verj- conspicuous in some shells, 

 called the Lunule ; on the other side is the elastic 

 ligament. The lunule, umbo, and ligament are in- 

 cluded within the dorsal or superior border ; the 

 opposite edge is termed the ventral or inferior 

 border. 



Fig. 28G0 is the same shell, its dorsal aspect dis- 

 played, showing the lunule, the umbo, the ligament, 

 and the thickness. 



Fig. 2861 exhibits the inside of the shell, showing 

 the lunule, the umbo, the hinge, or cardo, and the 

 ligament — the anterior muscular impression or mark 

 of attachment — the posterior muscular impression, 

 and the impression made by the edge of the mantle, 

 or pallial impression. 



As specimens of monomyarian shells we refer to 

 the prickly oyster (Spondylus), Fig. 2862, and to the 

 cocU's-eomb oyster (Ostrea crista galli). Fig. 2863. 

 The shell called Venus's Heart (Cardium caidissa), 

 Fig. 2864. one of the cockle tribe, is dimyariiui. 



Shut up in their shells, these mollusks have but 

 limited sources of animal enjoyment, and little com- 

 munication with objects around them. They grow 

 — they purify the waters of the sea— and afford food 

 to various creatures tenanting the briny deep, to 

 birds and quadrupeds haunting the shore, and to 

 man. Their nervous system is very simple ; they 

 have no organs of hearing, sight, or smell, yet would 

 it seem that their whole surface is influenced and 

 affected by light, by sounds or vibrations of the 

 water, by oJours, and liquid stimulants. It is as- 

 serted by fishermen that oysters in confined beds 

 may be seen, if the water is clear, to close their 

 shells whenever the shadow of a boat passes over 

 them. 



Though we say that these mollusks have no organs 

 of vision, we must not forget that certain brilliant 

 specks have been detected in thepecten, or scallop, 

 placed at short intervals round the thickened edge 

 of the mantle, and these. Poll and some other natu- 

 ralists regard as eyes ; if they be so, they are cer- 



tainly placed on the only part of the animal where 

 their use could be available. In Spondylus similar 

 specks have been observed. 



With respect to locomotion, these animals are 

 very limited. Some, however, as the fresh-water 

 mussel, are capable of raising themselves on the 

 edge of their half-opened valves, and of urging 

 themselves along, by the aid of a muscular organ 

 called the foot; and in this way they proceed by 

 successive impulses; and we have often watched 

 the fresh-water mussel thus proceed in shallow 

 clear water, leaving a long furrow in its track on the 

 soft mud over which it pushed its course. Some, 

 as the cockle, can leap a considerable height, and 

 clear the gunwale of a boat, and also burrow in the 

 sand by means of the same muscular instrument. 

 Others, as the scallop, by opening and flapping 

 together the valves, swim freely, with a rapi I de- 

 sultory movement ; and on the shore effect a back- 

 ward progression by the same action. Others 

 again, as the pholas, perforate rocks and masses of 

 chalk, in which they take" up their abode; and the 

 teredo pierces its way into the hulls of ships, float- 

 ing wood, the timbers of piers, of jetties, and of other 

 works which have cost man labour and capital. 



On the other hand, many bivalve mollusks are 

 firmly fixed to the rocks, or to other shells, by a 

 calcareous exudation that cements their shell to the 

 surface ; and, as in the oyster, clusters are often 

 compacted into large beds or masses. Others attach 

 themselves to the rocks by a cable or byssus, as the 

 common mussel and the pinna : this cable consists 

 of threads analogous to those formed by the silk- 

 worm and other caterpillare, or by the spider. The 

 threads exude, in a glutinous state, from a peculiar 

 organ at the base of the foot; they are not spun by 

 being drawn out, but are modelled, so to speak, by 

 the foot itself. If we examine the foot of a mussel 

 or of a pinna, we shall find a groove extending from 

 its root to its apex ; the edges of this groove fold 

 over, so as to form a minute canal. Along this 

 canal runs the glutinous matter, gradually becoming 

 tenacious. At the proper moment the animal pro- 

 trudes its foot, and its tip attaches the end of the 

 filament to the stone or rock. This done, it expands 

 the tongue-like foot, so as to open the canal and 

 free the filament from its sheath. The foot is then 

 withdrawn, new matter is poured along the groove, 

 and thus the operation is repeated till the cable is 

 secure. It is said that the pinna is capable of 

 producing only lour or five threads in twenty-four 

 licurs; the exudation and hardening of these 

 threads being a tedious process. 



Moored by their cable, these mollusks secure 

 themselves against the tide and the rolling of the 

 agitated waters. 



In Italy, gloves and other articles are manufac- 

 tured from the threads of the pinna, for the purpose 

 of being sold as curiosities. They may be seen in 

 most museums. 



We must not omit to state that the views respect- 

 ing the threads of the byssus which we have just 

 detailed, and which are generally received, ditt'er 

 materially from those of M. de Blainville. He does 

 not regard the byssus as the result of secretion, but 

 as an assemblage of muscular fibres, dried up in one 

 part of their extent, but still contractile and in a 

 living state at their origin, and that they were in 

 this condition throughout their whole length at the 

 time of their attachment to the rock. The tendinous 

 feet of byssoarca and tridacna seem to be, as he 

 conceives, a step towards the organization of a true 

 byssus. 



In pursuing our observations on the bivalve mol- 

 lusks, our remarks will be best understood if we 

 take a given species as the subject of our notice. 

 We select one with which all are larailiar, — viz. the 

 oyster; time immemorial one of the delicacies of 

 the table. The Greeks and Romans held these 

 "shell-fish" in great estimation; those of the Dar- 

 danelles, of Venice, of the Bay of Cumce, and of the 

 coasts of Britain, were the most esteemed. But the 

 Romans attached the greatest value to such as were 

 brought from these places and deposited in the 

 Lucrine Lake, where they grew very fat. Sergius 

 Orata, at Baiae, was the first Roman who enter- 

 tained the idea of modifying oysters into " natives" 

 by placing them in artificial oyster-beds. 



If we take one of these shell-fish, and carefully 

 open the valves of its shell, we shall observe a 

 broad free-floating membrane, continued from the 

 skin or solt integument. It appears in the form of 

 two outer leaves, one lining each valve of the 

 shell. These membranes, between which is the 

 body of the mollusk, constitute the mantle, or pal- 

 lium. Let us now turn back one of the leaves, or 

 lobes, as they are often termed, of the mantle, and 

 we shall .see two pairs of most delicale striated 

 lamellae, consisting of parallel fibres: these, com- 

 monly called the oyster's beard, are the gills, or 

 branchiae.* Between these two pairs of branchiae, 



• Ilcncc M. de Blainville'i terra for Uie Conchiffra, vix. Lamclli- 

 branvliiata. 



near the hinge, are slips or appendages, enclosing 

 I he mouth. We shall see also a firm, extensive 

 adductor muscle at the lower and outer side of the 

 body ; and above this a mass, consisting of the liver 

 and the viscera, the heart lying between the liver 

 and the inner side of the adductor muscle, in a 

 cavity or fissure, which is very ai)parent. Such are 

 the external appearances presented by the oy.iter, in 

 which no foot is developed, though it is slightly so 

 in the scallop, and greatly in many bivalves, as the 

 cockle, &c. 



The mouth is a simple orifice, bordered by four 

 long lips, near the hinge of the shell, and opens 

 almost immediately into the stomach, which, 

 through several orifices, receives the bile secreted 

 by the liver, a large mass of follicles loosely con- 

 nected together by a delicate tissue, and constitwt- 

 ing the epicurean morsel of this mollusk. The 

 alimentary canal, proceeding from the stomach, 

 winds through the liver, making a loop near the 

 heart. In some mollusks its couree is more com- 

 plex : in the cockle tribe, for instanee, it takes its 

 course through the substance of the foot, and in 

 others passes through the centre of the ventricle of 

 the heart. 



We need not here repeat that the gills or bran- 

 chiae are the aerating organs. In those creatures, 

 however, they serve also for another and very im- 

 portant purpose. It must strike every one who sees 

 an oyster, and considers how inertly it lies attached 

 to its native rock, or upon the bed whence it has 

 been taken, that some special means of procuring 

 food must be possessed, since the animal has neither 

 the power of following its prey nor of seeing and 

 seizing it ; and herein the branchiae fulfil a secondary 

 but most essential office. The water flowing into 

 the shell and traversing these branchiae contains 

 abundance of animalcules and animal and vegetable 

 particles, on which the oyster subsists. By the 

 action of minute cilia, to be seen only by means of 

 a microscope, by which the filaments of the bran- 

 chiae are thickly covered, strong and incessant cur- 

 rents are produced in the water, their course being 

 directed to the mouth, and with them the nutritious 

 particles on which the mollusk lives. The lips en- 

 folding the mouth are endowed with a sense of dis- 

 crimination, which rules them as to what particles 

 to reject and what to receive; and thus a constant 

 supply of food is obtained. The action of the cilia 

 is incessant, and goes on when the shell is closed, 

 agitating the water previously taken in. Nay, such 

 is their " visvitiE," that even when a portion of the 

 branchiae is cut away, they continue their move- 

 ments on the detached piece so long as their vitality 

 remains, and row it rapidly through the water, as if 

 it had an independent existence. 



The branchial filaments, which are very apparent, 

 are minute vessels running a parallel course, and 

 exposing the blood to the water ; they are enveloped 

 in delicate tissue, and, communicating with each 

 other, ultimately merge into two principal trunks, 

 conveying the renovated and purified blood to the 

 auricular cavity of the heart, whence it passes 

 through two canals to the ventricle, and is thence 

 distributed, by means of innumerable arterial rami- 

 fications, through the body. In some tribes there 

 are two auricular cavities, one for each pair of bran- 

 chiae ; and in Area there are two auricles and two 

 ventricles distinct on opposite sides of the body. 



To the office and structure of the mantle we may 

 now direct our attention. As in the gastropods, it 

 is the shell secreting and depositing agent. 



In the oyster the free margins of the mantle are 

 very limited, and are unconnected in any part of 

 their circumference with each other; but in other 

 tribes the mantle has its free edges more developed, 

 and the two leaves are more or less completely 

 united along their edges, so as to form an investing 

 cover, in which the body lies shrouded. 



In the mussel, for instance, the edges of the man- 

 tle are united with two orifices, one for the pro- 

 trusion of the foot, the other at the posterior ex- 

 tremity of the shell, forming a rudimentary siphon 

 for the rejection of the egcsta. In others again, as 

 the Charaidae, or clams, the circumference of the 

 mantle is united, leaving three apertures, one for 

 the protrusion of the foot, one for the entrance of 

 water to the branchiao, and one for the escape of 

 the egesta. 



In the Cardium, or cockle family, the mantle pro- 

 longs itself posteriorly intoadouble tube, or siphon; 

 and in the mactra this double siphon is very long, 

 exIenJing beyond the shell. Sometimes these 

 siphons are separate, sometimes conjoined. They 

 are eminently contractile, and fringed with papillae 

 of great sensibility. The uppersiphon is that through 

 which the excrementitious matters are thio.vn otf, 

 while the lower one is destined to convey water to 

 the branchiae. 



The Siphoniferous Bivalves, with long tubes, are 

 burrowing in their habits. By means of their foot 

 they scoop out a retreat in the sand or mud, and 

 hence it is that the manile is lengthened into a 



