Zoologists have taken advantage of certain modifications in the lobes of the mantle to establish divisions in their methodical arrange ments of the conchifera. This artificial means is sufficiently convenient, inasmuch as no anatomical inquiries are necessary in order to get at the distinguishing characters which these modifications supply. Latreille, in his 4 Fa milies du Regne Animal,' as well as other zoologists, have also made use of the conjunc tion or disunion of the lobes of the mantle to establish the principal divisions of their classifi cation ; but they have perhaps given too much consequence to these characters, inasmuch as they bear no relation to the number of the muscles. Nevertheless, none of the Mono myaria has yet been found which presents the lobes of the mantle conjoined, whilst the Dimyaria exhibit the two modifications which we have had occasion to mention, and which gives an opportunity to divide them into two grand series, the first comprising the whole of the Dintyaria whose mantles are united, the second all those whose mantles are open, or unconnected one lobe with another. The con chiferous Dimyaria which exhibit the lobes of the mantle united are modified in this respect in a remarkable manner, a circumstance which induces us to enter somewhat in detail into this part of the anatomy of the conchifera.
In making the series of acephalous mol lusca commence with those which have the lobes of the mantle completely distinct, we may place near them certain genera in which the branchix, conjoined in their posterior parts, form a kind of canal, within which the anus proceeds to terminate. This conjunction of the branchiw, extending as far as the edge of the mantle, forms a kind of band towards the pos terior commissure ; but, notwithstanding this, it may still be said that these animals have the lobes of the mantle altogether unconnected (Unio) (fig. 360); in other genera which have been held allied to this, the posterior band is not found, and already the lobes of the mantle appear united in thii posterior part, to a very small extent, leaving a particular perforation for the anus. The mantle still continues open in its circumference (Mytilus). By-and-by neighbouring genera, and even particular spe cies of the same genus, instead of a single per foration, present two (f, g, fig. 362) ; the second is destined to carry the water directly upon the branchim. When these two perfo rations have the faculty of being projected beyond the shell in the form of fleshy and con tractile tubes of various lengths, they have re ceived the special denomination of siphons; and the term perforation has been reserved to be applied to the holes of the mantle, which never pass the edges of the shell.
When the two siphons begin to appear, the lobes of the mantle still continue disjoined in a portion of their circumference; and this opening (b, b, fig. 356, h, fig. 362), is destined for the
passage of the foot.
In proportion as the foot is modified in its form, in proportion as it becomes more rudi mentary, the two lobes of the mantle are ob served in the succession of genera to become more and more extensively united, and it hap pens at length that in certain genera (Mya, Saxicava, &c.) a very minute submedian or anterior perforation, corresponding to the rudi mentary foot, is all that remains of separation between them. It is a circumstance worthy of remark that the siphons are observed to be come elongated and thickened in proportion as the lobes of the mantle are more extensively united. This circumstance, however, is only true in a general way, for it would be easy to quote many striking exceptions to it.
2. Siphons.—We have already had occasion to see the siphons commence in certain genera by simple perforations ; they increase in length in the succession of genera.; and in a certain number they always continue unconnected through their entire extent (g, h, fig. 346 ; b, c, fig. 355). In other genera, however, the siphons are seen at first united towards their base, then conjoined nearly to the middle, co hering almost to their ends, and finally blended through their whole length, so as to form a single elongated subcylindrical fleshy mass, pierced through its entire length by the canals of the two siphons, one of smaller size, situ ated superiorly for the anus, the other larger, situated under the former, and destined to transmit the water to the branchiw. Whether connected or not, the superior siphon is always characterized as the anal, the inferior as the branchial siphon.
The structure of the siphons is entirely mus cular, so that their free extremities are capable of contracting and of being elongated to a very considerable degree. 'They are beset around their external orifices with a great number of papillw, (n, o, fig. 347), occasionally truncated at their extremities and of exquisite sensibility. The water has to pass over these papillw before it can enter the mantle, and un doubtedly they apprise the animal of the pre sence of every foreign body that might injure it. In a few genera the siphons contract by means of their component muscular fibres; but in the greater number they have a parti cular retractor muscle running on each side of the animal, and in relation, in point of mag nitude, &c. with the length and degree of con tractility possessed by the siphons (p, fig. 347). The existence of this muscle, and consequently of siphons, is manifested on the interior of the shell by a posterior sinuous furrow of various depth, and indicating upon a narrow line the point of implantation of the retractor muscle of the siphons.