This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
(1323). The Cockle family (Cardiacea) is recognized by having the mantle open anteriorly, but prolonged at one extremity into two tubes, one of which admits the water for respiration, while the other discharges effete matter. In the Cockle (Cardium) the tubes are short, and scarcely reach beyond the shell (fig. 252, a); but in other genera, as, for example, Mactra (fig. 250, b, c), they are of such length, that, when extended, they protrude to a considerable distance. We at once perceive the use of the tubular arrangement of the mantle here referred to, when we reflect upon the already-mentioned habits of this extensive division of the Conchifera, and consider how, by means of their largely-developed foot, they burrow into the sand or mud of the shore. Had their mantle been open, like that of the Oyster, respiration would have been impossible under the circumstances in which they live; but, by the modification of structure thus provided, their tubes being prolonged to the mouth of the excavation wherein they reside, water is freely admitted to the branchiae through one of the passages so formed, and excrement ejected through the other (fig. 251).
Fig. 250. Mactra: a, foot; b, c, siphons formed by the mantle.
(1324). Whoever watches these siphoniferous bivalves in a living state will readily appreciate the importance of the pallial prolongations forming this tubular apparatus; especially if minute floating particles are placed in the water wherein they are confined. It will then be perceived that powerful currents are perpetually rushing through the extremities of each siphon, caused by the rapid action of cilia placed within; and the streams thus produced not only form a provision for constantly changing the water in which the branchiae (fig. 251, g) are immersed, but forcibly convey floating molecules to the aperture of the mouth, which is situated in the position indicated in the figure by. the letter h, and thus supply abundance of nutritive materials that could, apparently, in animals so destitute of prehensile organs, have been procured by no other contrivance*.
Fig. 251. Mactra: a, b, siphons; e, hinge; c, d, adductor muscles; f, foot.
(1325). The last family of this class includes those species which, like the Pholas and Teredo, bore in stone or wood, or, like the Solen, penetrate deeply into the sand. In such, the mantle is prolonged into terminal tubes of great length, and their shells remain always open at the extremities: these constitute the division to which Cuvier has applied the name "Enfermes," on account of the very complete union of the two sides of the mantle; and from such forms of Concuifera the transition to the Tunicata, described in the last chapter, is by no means difficult.
(1326). In animals circumstanced as the Conchifera, it would be vain to expect any high development of the nervous system, or senses of an elevated character; nevertheless a few small ganglia are perceptible in different parts, and nervous threads of extreme tenuity are seen to arise from them and to be distributed in various directions.
(1327). One pair of ganglia, in the Dimyaria, is easily distinguished, occupying the ordinary position of the brain, namely above the oesophagus. Hence is derived a supply of nerves to the sensitive labial appendages, to the oral orifice, and other neighbouring parts. Two other ganglionic masses, of larger size than the brains properly so called, are placed near the posterior retractor muscle; and a fifth small ganglion, in those species provided with siphons, is found in the vicinity of the breathing-tube, the muscular walls of which receive nerves from this source.
(1328). In the Monomyaria the nervous centres are still more feebly developed, and the posterior ganglia proportionately smaller than those found in species possessed of two adductor muscles.
(1329). No organ of sense, other than those already noticed, is met with in any of the Conchifera, except in one remarkable instance. In the Scallops (Pectin) the edges of the mantle are studded with numerous pearl-like points, interspersed among the retractile tentacula placed around its circumference. These, which are represented in the figure of Pecten already given (fig. 247), are considered by Poli1 to be so many distinct eyes thus singularly situated; and, from the circumstance of their being furnished with so many organs of vision, he applied the name of Argus to the mollusca possessing them. Should the brilliant specks in question be really ocelli, they certainly are placed in the only position where they can be efficient as instruments of sight, inasmuch as the margin of the mantle is, in such animals, the only portion of the body capable of being protruded beyond the boundaries of the shell to a sufficient distance to allow the creature to peep into the world around it.
* The parte represented in the above figure (fig. 251) which are not particularly pointed out in the text are the anterior adductor muscle (c), the posterior adductor muscle (d), the elastic ligament of the hinge (e), and the largely-developed foot (f).
1 Testacea utriusque Siciliae, eorumque Historia et Anatome: 3 vols. fol.
(1330). The elaborate researches of M. Siebold have demonstrated the existence of another sense in the Conchiferous Mollusca, namely that of hearing - or at least have pointed out the presence of an organ which from its structure seems to be appropriated to the reception of sonorous impressions. This remarkable apparatus is situated in the foot, and is thus described* by its discoverer as it occurs in Cyclas cornea: - "On compressing the extremity of the foot of this species between two plates of glass, we bring into view a large central nervous ganglion, and on each side of this there is a minute round reservoir, composed of an elastic, opake, and tenacious substance. In the centre of this is contained a perfectly transparent circular and flattened nucleus, which floats disconnected from the sides of the body that contains it, and has an oscillatory movement. This nucleus appears to consist of a crystalline salt".