mollusks, being found also in some Gasteropods, as, e. g. in certain species of Strombus, in Trochus turritus, and in a species of Murex.*

Our knowledge of the food of bivalves may be considered as almost entirely conjectural. It seems, however, to have been ascertained, that oysters feed upon infusory vegetables and animalcules; and, as it has been asserted, that while various species of these are beneficial, others are actually injurious, it seems to follow that oysters must be able to distinguish and reject the latter; and the organs of discrimination are undoubtedly the sensitive labial palps that guard the oral aperture. The green colour which oysters acquire in their parks of brackish water, is owing to the colour of the animalcules that furnish them their food; and hence M. Gaillon has remarked that other animalcules communicate to the oysters a colour similar to their own, tinting them or brown, or grey, or yellowish.†

Other bivalves are probably nourished by similar animalcules; for, when we reflect on their apparently helpless and inert condition, hampered with their shells, or even bound to the rock, we cannot but perceive that they are all unfit for the capture of any other prey than what floats about and within them. And how abundantly is this furnished! There are everywhere scattered on the bed of the wide ocean extensive beds of oysters, clams, mussels, &c., containing millions of individuals, which are hourly devouring, each of them, crowds of animalcules (embracing in the term the infusory, microscopic, crustaceous and gelatinous medusa), which, from their vast numbers and rapid reproduction, never fail them. At some seasons of the year I have seen the waters of our shores literally in a move with Entomostraca; and I am fully satisfied that, when Scoresby calculated a cubical mile to contain 23,888,000,000,000,000, he was not exaggerating the actual fact. In one family of *Edinb. New Phil. Journ. vii. 231.

Edinb. New Phil. Journ. iv. 196.-The excrement of oysters has given rise to a curious simile; "And though some count a jesting lie to be like the dirt of oysters, which (they say) never stains, yet is it a sin in earnest."-FULLER'S Holy and Profane State, p. 379.

"The number of Medusa in the olive-green sea was found to be immense. They were about one fourth of an inch asunder. In this proportion, a cubic inch of water must contain 64; a cubic foot, 110,592'; a cubic fathom, 23,887,872; and a cubical mile about 23,888,000,000,000,000! From soundings made in the situation where these animals were found, it is probable the sea is upwards of a mile in depth; but whether these substances occupy the whole depth is uncertain. Provided, however, the depth to which they extend be but 250 fathoms, the above immense number of one species may occur in a space of two miles square. It may

bivalves furnished with a byssus, we frequently find entangled amid its fibres, or concealed within the valves, one or more small crabs (Pinnoteres), of which the older naturalists, who never left an observation to stand, like truth, all naked, but ever clothed it with some pretty vestment, tell us a tale not to be passed over in this place, and which I present you in the words of Dr. Philemon Holland, the laborious translator of Pliny. "The Nacre, also called Pinnæ, is of the kind of shell fishes. It is alwaies found and caught in muddie places, but never without a companion, which they cal Pinnoter, or Pinnophylax. And it is no other but a little shrimpe, or, in some places, the smallest crab, which beareth the Nacre companie, and waites vpon him for to get some victuals. The nature of the Nacre is to gape wide, and sheweth vnto the little fishes her seelie body, without any eie at all. They come leaping by & by close vnto her; and seeing they haue good leaue, grow so hardie & bold, as to skip into her shel and fill it ful. The shrimp lying in spiall, seeing this good time & opportunitie, giueth token thereof to the Nacre, secretly with a little pinch. She hath no sooner this signall, but she shuts her mouth, & whatsoever was within, crushes & kills it presently; and then she deuides the bootie with the little crab or shrimp, her sentinell and companion. I maruell therefore so much the more at them who are of opinion, that fishes and beasts in the water haue no sense.

Of bivalves there are some which, as I have told you, bore into wood and rocks; but I need scarcely guard you against entertaining the supposition that they eat the material on which they work, although there are authors who have attributed to them "a stone-eating power and appetite." The Teredines, however, really eat the wood destroyed by them; for Mr. Hatchett proved the pulp in

give a better conception of the amount of Medusa in this extent, if we calculate the length of time that would be requisite, with a certain number of persons, for counting this number. Allowing that one person could count 1,000,000 in seven days, which is barely possible, it would have required that 80,000 persons should have started at the creation of the world, to complete the enumeration at the present time! What a stupendous idea this fact gives of the immensity of creation, and of the bounty of Divine Providence, in furnishing such a profusion of life in a region so remote from the habitations of men! But if the number of animals in a space of two miles square be so great, what must be the amount requisite for the discoloration of the sea, through an extent of perhaps 20,000 or 30,000 square miles!"-SCORESBY'S Arctic Regions, i. 179.-See also Darwin's Journal, iii. 14, &c.

Hist. of the World, i. 261.

Fig. 55.

their intestine to be vegetable sawdust; but I agree with Sir E. Home in thinking (as Sellius, indeed, long before asserted), that the sawdust serves only as a substance in which the real food. procured from the sea is entangled and prevented from escaping too readily from the stomach. I will give you Sir Everard's description of the digestive organs of these animals, which a comparison will prove to be altogether different from those of the more typical bivalved mollusca. The oesophagus (Fig. 55, a) is now very short, and lies on the left side of the neck the canal swells out, and becomes stomach (b), which, in its external appearance, is a large bag, extending the whole length of the cavity of the abdomen, but when laid open it is found to have a septum (c) dividing it longitudinally into two equal portions, except at the lowest part, where they communicate (d), the septum being wanting. The intestine has its origin close to the termination of the œsophagus, is extremely small, dilates into a cavity containing a hard white spherical body the size of a pin's head, and then makes a turn upon itself. The course it follows is shown by the letters e in the cut.

d

* This figure represents the course of the stomach and intestines of Teredo navalis, removed from the body. a, The oesophagus; b, the stomach; c, the septum, dividing it into two cavities; d, the aperture by which the two cavities of the stomach communicate; e, the course of the intestine to its termination.-Comp. Anat. t. 80.

+ See Hancock in Ann. and Mag. N. Hist. ser. 2. ii. 232. Home's Comp. Anat. i. 373.

305

LETTER XVIII.

THE CARNIVOROUS MOLLUSCA.

ALTHOUGH it may be true, as stated in the preceding letter, that the great proportion of the Conchifera subsist on infusorial entities, or on food in a state of molecular division, yet there can be no doubt that some of the larger and locomotive species seek a more substantial fare, and feed on worms or other animal matter in a state of partial decay; which they seem to have the power of grasping by means of their extensible labial appendages. Thus the large Cyprina islandica and the Modiola vulgaris of our seas often swallow the bait of the fisherman; and in the stomach of an individual of the former I once found the undigested remains of a large green Nereis enveloped in a pulp too consistent certainly to have been the sediment from water, however loaded with molecules.

In their manner of feeding, these Conchifera resemble the pectinibranchial Gasteropods whose shells have a notch or canal at the base of their apertures; and it is important you should remember that it is only, with a few exceptions, the Gasteropods of this order (Pectinibranchia) so circumstanced that are truly carnivorous. They embrace the cowries, the cones, the volutes, the rock shells and the whelks, all of which live on animal food; and it seems to be indifferent to them whether their prey is dead, or still fresh and alive; but, in the latter case, it is obvious, if you remember the inactivity, and sluggishness, and total want of cunning of these molluscs, that the prey they can master must be fettered and stationary, or endowed with locomotive powers and arms not superior to their own. It is not unlikely that they may prefer a dead prey to a living one, for we know that the whelks will take a bait readily; in search of which they frequently enter the baskets laid for crabs and lobsters, which are always baited with garbage; while in tropical climes we are told that men fish for the olives with lines, to which small nooses, each containing a piece of the arms of a cuttlefish, are appended.

You could never have anticipated that the Bivalved Mollusca (Conchifera) would be found among the prey of these

X

carnivorous tribes, than which there are apparently no animals less fitted to gain access to their strong-holds, so that even Blainville has expressed himself incredulous on the point. But the fact is certain, and has been known since the time of Aristotle; nor, indeed, is it hastily to be believed that such an improbable statement would have been made by the Stagyrite, had it not rested on his personal observation. The Purpuræ prove extensively destructive to mussels and other littoral bivalves: the Buccina feed upon those which burrow in sand in somewhat deeper water; and it is very probable, considering the similarity of their organisation, that all the whelks and rock-shells, and perhaps all the pectinibranchial zoophagous Gasteropods, have the same taste, and an equal capacity of gratifying it.

How, you

ask, and by what means? Do they glide insidiously, and pop a stone between the valves, to prevent their closure? or do they venture slily to insinuate their foot, and seize upon the unwary inmate? The first they cannot do, and the latter I should deem a hazardous attempt; but nevertheless it is affirmed that the Buccinum undatum really runs the hazard in its attacks upon the Clam (Pecten opercularis), to which it bears a great enmity. This is not, however, their usual method, which is what you might never guess-by boring a hole in one valve, through which they reach their miserable victim. On examining a number of valves of dead shells, of Mactræ and Anatinæ especially, you will perceive in many, and generally near the beaks, a small circular hole drilled with a neatness that the gimlet of the artisan could not more than emulate; and these holes are the workman

* As Rondeletius did before him. He maintains that the Purpuræ can only draw out and suck the snails. Hist. des Poiss. ii. 45.

Hist. Anim. lib. iv. cap. iv. sec. 148-9.

"Is commonly taken in dredging by fishermen, who either use the animal for bait, or destroy it, from a supposition that it is very destructive to the Large Scollop, Pecten maximus, by insinuating its tail (as it is termed) into the shell, and destroying the inhabitant: this, we have been assured, they will do even in a pail of sea-water." MONT. Test. Brit. p. 238. The mode in which they anciently fished for the Purpuræ proves the danger. "Now these purples are taken with small nets, and thinne wrought, cast into the deep; within which, for a bait to bite at, there must be certain winckles and cockles, that will shut and open, and be ready to snap, such as we see those limpens be, called mituli. Halfe dead they should be first, that, being new put into the sea again, and desirous to revive and live, they might gape for water and then the purples make at them with their pointed tongues, which they thrust out to annoy them; but the other, feeling themselves pricked therewith, presently shut their shells together, and bite hard. Thus the purples, for their greedinesse, are caught and taken up, hanging by their tongues." HOLLAND'S Plin. i. 259.