168 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



of which it is composed, although of such extreme delicacy, is of great 

 permanence, and does not exhibit the slightest tendency to decay, how- 

 ever long it is preserved ; so that, when once consolidated, it appears 

 to undergo no further change in the living animal. The growth of the 

 animal, however, requires a corresponding enlargement of its enveloping 

 shell ; and this is provided for by the simple process of superficial de- 

 posit, through the subdivision of the whole shell into component plates. 

 For by the addition of new matter at the edge of each plate, by the 

 consolidation of a portion of the soft membrane that intervenes between 

 the adjacent plates, the whole shell is enlarged, without losing its 

 globular form. At the same.time it is strengthened in a corresponding 

 degree, by the consolidation of the soft tissue at the surface of each 

 plate. And, in like manner, the spines are enlarged and lengthened 

 by the progressive formation of new layers, each on the exterior of the 

 preceding ; so that a transverse section exhibits a number of concentric 

 rings, like those of an Exogenous tree. Thus even in the growth of 

 this complex and elaborate structure, we recognise the principle of 

 superficial deposit, which we shall find to be universal amongst the hard 

 parts of the Invertebrata : notwithstanding that, at first sight, it would 

 have appeared impossible to provide on this plan for the gradual en- 

 largement of a globular shell, completely enclosing the animal, and 

 therefore required to keep pace with the latter in its rate of increase. 



280. Among the Mollusca, we find the body sometimes altogether 

 destitute of solid organs of support, protection, or locomotion, as is the 

 case, for example, in the Slug ; and the movements are feeble and the 

 habits inert, the muscles having no fixed points for their attachment, 

 and acting without any of the advantages of leverage. In other cases, 

 we find the body more or less completely protected by a Shell ; which 

 is sufficiently large in some instances to cover the body completely, 

 whilst in others it affords only a partial investment. The plan on which 

 this shell is formed, however, is very different from that which has just 

 been described; being much less complex. The Univalve shells, or 

 those formed in one piece, are always of a conical form ; the cone being 

 sometimes simple, as in the Limpet ; in other cases being spirally coiled, 

 as in the Snail. Now the base of this cone is open ; and through this, 

 the animal can project its movable parts. When its increasing size 

 requires additional accommodation, it is obvious that an addition to the 

 large end of the cone will increase its diameter and its length at the 

 same time ; so as to afford the required space, without any alteration in 

 the form or dimensions of the older and smaller portions of the cone. 

 This last, indeed, is frequently quitted by the animal, and remains 

 empty ; being sometimes separated from the later portions, by one or 

 more partitions thrown across by the animal, as is seen especially in 

 the Nautilus and other chambered shells. Besides the new matter 

 added to the mouth of the shell, a thin layer is usually formed over its 

 whole interior surface ; so that the lining of the new part is continuous 

 with that of the old. In the Bivalve shells, we trace this mode of in- 

 crease without any difficulty ; especially in such shells as that of the 

 Oyster, in which the successive laminae remain distinct. Each lamina 

 is interior to the preceding, being formed on the living surface of the 



