. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Contractile cilia magnified. distributed some to the edges of the mantle, and others to the tentacula with which it is fringed. The whole of these parts are extremely retractile, and are endowed with such sensi- bility that the slightest contact is perceived, as is evinced by their instantaneous contraction. 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, inasm
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Contractile cilia magnified. distributed some to the edges of the mantle, and others to the tentacula with which it is fringed. The whole of these parts are extremely retractile, and are endowed with such sensi- bility that the slightest contact is perceived, as is evinced by their instantaneous contraction. 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 ' 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 Dimyaria 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 branchiae, conjoined in their posterior parts, form a kind of canal, within which the
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