Archive image from page 722 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana01todd Year: 1836 CONCHIFERA. 707 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. Sip/ions.—We have already had occasion to see the siphons commence in certain genera by simple perforati6ns; they increase in length in
Archive image from page 722 of The cyclopædia of anatomy and. The cyclopædia of anatomy and physiology cyclopdiaofana01todd Year: 1836 CONCHIFERA. 707 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. Sip/ions.—We have already had occasion to see the siphons commence in certain genera by simple perforati6ns; they increase in length in the succession of genera; and in a certain number they always continue unconnected through their entire extent (g, h, Jig- 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 branchiae. 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 papillae, (n, o,fg. 347), occasionally truncated at their extremities and of exquisite sensibility. The water has to pass over these papillae 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- cula
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