1TO 



STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



Teeth, which we shall presently find to be formed upon the very same 

 plan ( 318). 



282. We are to regard this kind of shell-substance, therefore, as 

 formed by the secreting action of the epithelial cells covering the mantle 

 of the animal, which membrane, though it answers in position to the 

 skin, has the soft, spongy, glandular character of a mucous membrane. 

 These draw calcareous matter into their cavities, as a part of their own 

 process of growth : this matter being supplied from the fluids of the 

 vascular surface beneath. Now when these calcigerous cells are sepa- 

 rated by intercellular substance, they remain distinct through the whole 

 of their lives, and they form by their cohesion a tenacious membrane, 

 that retains its consistency after .the removal of the calcareous matter. 

 But this is only the case in certain groups of shells, chiefly belonging to 

 the bivalve division. When the intercellular substance is wanting, and 

 the cells come into close contact, their partitions become indistinct on 

 account of their extreme tenuity ; and not unfrequently a fusion of the 

 whole substance appears to take place, by the dissolution of the original 

 cell-walls, so that it becomes more or less homogeneous, traces of the 

 original cellular structure being here and there distinguishable ( 252). 



283. Sometimes where this fusion has taken place, so as to obliterate 

 the original cell-structure, we find the almost homogeneous substance 

 traversed by a series of tubuli, not arranged, however, in any very defi- 

 nite direction, but forming an irregular network (Fig. 44). These tubes 

 vary in size from l-2000th to l-20,000th of an inch; but their general diame- 

 ter, in the shells in which they most abound, is 1 -4500th of an inch. In 

 the larger tubuli, something of a bead-like structure may occasionally be 

 seen : as if their interior were occupied by rounded granules arranged 

 in a linear direction. Although it might be supposed that this structure 

 is destined to convey nutrient fluid into the substance of the shell, yet 

 there is no evidence that such is the fact ; and, on the contrary, there is 

 ample evidence that, even in sh ells most copiously traversed by these tubuli, 

 no processes of interstitial growth or renewal take place. The perma- 

 nent character of the substance of all Shells, when once it is fully formed, 

 is as remarkable as that of Coral : and as the adaptation of their size, 



Fig. 44. 



Tubular shell-structure, from Anomia. 



to that of the animals to which they belong, is entirely effected by addi- 

 tions to their surfaces and edges, no interstitial deposit can have a share 

 in producing it. 



