BRACHIOTODA (Gr. arm-footed), or (Gr. mantle-gilled), a class of molluscous animals, having hivabie shells, but differing in important points from the vast majority of recent mollusks with bivalve shells, the math-ranch ia ta (q.v.). The chief differences existing in the smelly itself have been already pointed out in the article bivalve shells (q.v.), but those of internal structure arc still more impor tant. The mantle or pallium (see MOLLUSCA) in the B. consists of two broadexpansions or lobes, covered by the two valves of the shell, and inclosing all the other soft ports of the animal; whilst respiration or the Oration of the blood is carried on by the surface of these lobes themselves, traversed by minutely ramifying blood-vessels, extended into processes, and furnished, especially along the edge, with vibratile cilia which create a continual current in the surrounding water, and thus keep up a fresh supply, front which the necessary air may be obtained. The organs by which food is procured are also remarkable—two long arms arising from the sides of the month, and disposed wholly or partly in spiral curves, when not extended to seek or seize prey. These arms
are usually furnished with numerous vibratory filaments, which are supposed not only to aid in the capture of prey, but in the maintenance of the current necessary for respira tion. The B. are attached to solid bodies either by a footstalk or by one of the valves of the shell. Of existing species, the terebratulre or lamp-shells (q.v.) are by far the most numerous; but even these appear to have existed in far greater numbers in former geologic periods, and of some of the other families of B. only a single species is known to exist, or the existing species are very few, whilst ,the fossil species are very numer ous. The existing species are very widely diffused over the globe. All of them are marine, and one (crania personata) has been brought up from the depth of 25.5 fathoms. The B. are regarded as exhibiting structural affinities not only to the urcidia (q.v.) and the lairtellibranchiata, between which they are commonly placed, bat also to the class bryozoa or polyzoa among zoophytes (q.v.).