416 INCLUSA ;-PHOLAS, TEREDO. being distended with water, the animal suddenly contracts it; and thus a jet is produced through the anterior orifice, which washes out the part of the cavity occupied by the animal; but, as many of the particles expelled by it are deposited before they reach the mouth of the hole, the passage is found to be lined nearer its entrance with a soft mud. 960. The Pholades evidently prefer such beds, as are composed of indurated clay, or soft lime-stones, to harder lime-stones; though they are occasionally found in the latter. Hence it might be supposed that their action is always of the mechanical nature just described; but though it is certainly of this kind in many instances (as is proved by the cylindrical form of the perforation, as also by the rasp-marks on its interior), Pholades are sometimes found so imbedded, that they could not have turned in their perforation.-In some countries they are much prized as food; especially along the Mediterranean coast. They possess one remarkable property, which is not, however, confined to them, but which is manifested in a degree by other Mollusca ; this is their phosphorescence. It is said to be so strong, that persons who eat them raw, and in an obscure or dark place, seem to be swallowing phosphorus. The Pholades are pretty generally diffused over the globe. They usually multiply around any spot which they have begun to frequent; the young produced from the eggs probably boring near their parents. Pholades are not very abundant in a fossil state; but they are occasionally found in tertiary strata, imbedded in the cavities. which they have themselves formed. 961. The Teredo is a genus which presents many points of interest to the Naturalist, as well as to those who are practically affected by its destructive operations. This animal bears a general resemblance to the Pholas; and it carries on its anterior part a pair of valves, which it uses in the same manner, as perforators of the wood into which it bores. When quite young, it establishes its habitation in submerged timber, such as ships' bottoms, piles, &c., which it perforates in every direction. With its increasing bulk, it enlarges its hole, advancing into the wood; but it does not draw the tubes of the mantle after it, for they remain where they were, and deposit shelly matter, which INCLUSA TEREDO OR WOOD-WORM. 417 lines the cavity; and thus a complete additional tube of shell is formed, of a length proportional to the age of the animal. At the entrance of the tube, or the termination of the siphons, there is a pair of pallettes, or small valves of shelly structure; by the motion of which the current of water is maintained, that is necessary to bring a supply of food and oxygen to the animal thus included. The highest point at which they commence to bore, is always some feet below the lowest water-mark; and they usually work downwards. The orifice, being made when the animal was young, is very small, and often difficult to perceive. The beginning of the tube is usually horizontal or oblique; and afterwards it curves into a nearly vertical direction. The nature of the wood has a great influence on the regularity and direction of the canal which is hollowed in its interior; but this is still more affected by the neighbourhood of other Teredines, to avoid whose tubes the animal will make sudden curves. It is difficult to comprehend how it can become conscious of their proximity. There can be no doubt that, in this instance, the excavation is effected by the valves; since it is always cylindrical, with very smooth walls; and the shell is adapted, both by its sides and edges, for rasping and boring. 962. These animals are among the most formidable destroyers of the works of Man. When it is considered that their instinct prompts them to attack all the timber which he has, for various objects, placed beneath the surface of the sea, it is seen that the field of their operation is immense. The piles of bridges, piers, and harbours, as well as shipping, are liable to their devastations; and Holland has been more than once threatened with an inundation, by the destruction of the dykes which they have effected. Many vessels have sprung leaks and foundered, owing to the unsuspected demolition of the planking of their bottoms by the same means. The most effectual protection against their attacks, is a metallic sheathing; but it is said that piles may be secured by the previous charring of their surface to the depth of a few lines. Different species of Teredo appear to exist in all The accessory pieces of the shell of the Pholas, may probably be considered as a rudiment of the same structure; they vary considerably in number and arrangement; and sometimes appear to form the commencement of a regular tube. 418 INCLUSA; -GASTROCHÆNA, CLAVAGELLA, ASPERGILLUM. parts of the world; and on the coasts of the Atlantic they are used as food, their flesh being reputed more delicate than that of Oysters. According to Seba, who made his observations in Holland, certain species of Nereis (§ 839) are mortal enemies to the Teredines, penetrating into their tubes and devouring them. As a British animal, the Teredo is now nearly or quite extinct ; the precautions taken against it having prevented its multiplication; and new importations being checked by the general use of copper sheathing. 963. In the Gastrochana, the valves bear but a still smaller proportion to the shelly tube, which not only covers the part of the excavation posterior to the animal, but lines the whole interior of the hollow, so as to include the valves. This tube is often found in the perforations previously made by some other boring Mollusk, and also in natural hollows in rocks, corals, &c.; but the animal has evidently the power of exca vating for itself, if it does not meet with a hole adapted to it. The Clavagella is an animal of analogous structure; but one of its valves is incorporated, as it were, with the shelly tube, the other remaining free. The latter appears to perform by its movements the same kind of function as the pallettes of the Teredo.-The Aspergillum departs more widely from the general type, than any of the preceding; so that, if viewed out of connexion with them, its true place in the scale would be doubtful. The shell, which derives its name from its resemblance to the spout of a watering-pot, has the form of an elongated cone, terminating at the large end in a disk, which is pierced with a number of tubular orifices; and the tubes of the outer row being the longest, they form a sort of corolla (a, Fig. 597) around the disk. At a little distance above this, two small valves (b), incorporated in the substance of the tube, are easily distinguished. The smaller end is open, and there is likewise a little fissure a FIG. 598.-ASPER GILLUM. ASPERGILLUM.-GEOLOGICAL DISTRIBUTION. 419 nearer the larger end; by these apertures the water is freely admitted to the interior of the shell. The animals of this genus are borers. Some of them live in the sand, plunged down perpendicularly for about three-fourths of their length, and supported by the little tubular prolongations, which are supposed to be filled by fleshy filaments of the mantle. Some, again, burrow in stone, others in wood, and others in thick shells. 964. In regard to the Geological distribution of the Lamellibranchiata, it will be sufficient (as in the case of the Gasteropoda) to say, that they make their appearance in the earliest fossiliferous strata; and that, although there is always a difference between the fossil and the existing species, until we compare those of the comparatively recent geological periods, the same genera are found to have existed in the ocean, from the most ancient times to the present. It is interesting to remark, however, that the proportion which they bore to the Bivalves of the succeeding class, was very small in the oldest fossiliferous rocks, but has since been gradually reversed. CHAPTER XIX. OF THE CLASS OF PALLIOBRANCHIATA. 965. THIS Class, although at present very limited,--both as to the number of existing species it includes, and the small number of these which seem to be distributed through the ocean,—was formerly of great importance; since it included a very large proportion of the Bivalve Mollusks of the older rocks. Although an ordinary observer would not detect anything in the structure of the shell, to justify the separation of this class as distinct from the preceding, yet the necessity for this division becomes evident, when the organisation of the animal is examined. The name of the Class is derived from the peculiar conformation of the respiratory apparatus, which here consists of the mantle itself; this is traversed by blood-vessels, which ramify minutely over its surface; and is furnished, especially along its edge, with vibratile cilia, which produce a continual current in the surrounding water, and thus renew it for the aeration of the circulating fluid. In addition to this singularity of structure, the organs for procuring food offer remarkable peculiarities. They consist of two long spiral arms, one on each side of the mouth; the existence of which has given rise to the name BRACHIOPODA or Arm-footed, by which the class is frequently designated. In many species, these are capable of not only being unrolled, but extended beyond the shell to a great distance in quest of food. They are usually furnished with numerous vibratory filaments, for the more certain capture of the prey, and probably also for assisting in the maintenance of the respiratory current an extraordinary provision, which is rendered necessary by the great depth at which these animals live, and the consequent enormous pressure of the water around. All the existing genera of this class are attached, in some way or other, to solid bodies. In Terebratula and |