. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Tubular shell-structure from external surface of Anomia Ephippium. Magnified 250 diameters. most abundant in shells whose exterior has a foliated or sculptured character; and not un- frequently they may be distinctly seen to pass directly towards the prominences of the sur- face, — as in Lima scabra and various species of Chama. They are by no means restricted, however, to shells thus characterised; nor are they universally present in them. Of the origin and mode of formation of the membranous shell-structure, it is rath
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. Tubular shell-structure from external surface of Anomia Ephippium. Magnified 250 diameters. most abundant in shells whose exterior has a foliated or sculptured character; and not un- frequently they may be distinctly seen to pass directly towards the prominences of the sur- face, — as in Lima scabra and various species of Chama. They are by no means restricted, however, to shells thus characterised; nor are they universally present in them. Of the origin and mode of formation of the membranous shell-structure, it is rather diffi- cult to give an exact account. Possibly, after the epidermic cells have undergone calcifica- tion, so as to form the external cellular layer, the basement membrane itself may become detached from the surface of the mantle, in combination with a layer of calcareous matter. Even in nacre, however, which may be con- sidered as the most perfect form of this sub- stance, indications of cellular structure are not unfrequently to be seen, especially in univalve shells : these are particularly evident in Haliotix, the nacreous laminae of which, when carefully examined under a sufficiently high magnifying power, are found to be com- posed of minute cells of a long oval form (fig. 416.), their short diameter not bein<* above l-5000th of an inch. Their boundaries in many parts are very indistinct, or even disappear altogether, so that every gradation can he traced, from the obviously cellular ar- rangement shown in fig. 412., to the homo- geneous aspect presented by the nacre of bivalve shells. The same cellular structure, and the same gradation to a homogeneous stratum, may be made apparent in the decal- cified membrane ; so that here we seem to have evidence that even the membranous shell- substance is originally formed by the agency of cells, although the boundaries of these have usually been subsequently obliterated, so that the structure comes to present a homoge- neo
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