488 



TEGUMENTARY ORGANS. 



their hardness to a generally diffused, trans- 

 parent, calcareous deposit, which allows the 

 previous dotted structure of the laminae 

 to be perfectly obvious. In some parts and in 

 the superficial layers, this deposit is struc- 

 tureless and homogeneous (Jig. 312. G, a), but 

 in other parts the youngest layer presents very 

 delicate polygonal meshes, whose areae were 

 about T oVo i n - m diameter (fig. 312. F, a). De- 

 calcification completely destroys this appear- 

 ance ; so that I imagine it to be caused merely 

 by the mode in which the primary deposit in the 

 membrane takes place, the areolae becoming 

 almost immediately fused together by further 

 deposit. 



Through these homogeneous hardened outer 

 layers thus constituted, there are dispersed 

 more opaque spots (Jig. 312. F, G, b), more 

 or less rounded in their outline, and varying 

 in diameter from -^ in. or less, to ten times 

 that size. The smallest of these bodies have 

 exactly the appearance of cells (fig. 312. F, 6), 

 consisting of a dark centre, with a circular more 

 transparent wall, and every variety of form 

 may be observed between these and large 

 masses, such as that figured {fig. 312. G, 6), 

 with a lobulated laminated circumference, and 

 an irregular centre, composed of small masses 

 like dentine globules. In the former the dots 

 of the original tissue may be still seen ; but 

 in the latter they are not traceable and seem 

 to be obliterated. If dilute hydrochloric acid 

 be added while the object is still under the 

 microscope, however, these bodies are gra- 

 dually dissolved out with eflfervescence,andthe 

 structure of the place they occupied is found 

 to be identical with that of the other portions 

 of the integument. They are, therefore, nothing 

 but concretions of calcareous matter, whose 

 deposit has taken place in a peculiar form, 

 quite independently of the primary structure 

 of the part ; this form being, in the smaller 

 concretions, most deceptively cell-like. It 

 appears to me that this case, in which the 

 assumption of structure without cell develop- 

 ment may be so plainly demonstrated, has a 

 most important application, not only to the 

 mode of formation of Crustacean and Mol- 

 luscan shells (vide infra), but to the develop- 

 ment of the teeth, strongly confirming, I think, 

 the view which I have taken of that pro- 

 cess. 



Integument of the Mollusca. The soft surface 

 of the body of the Mollusca in general is con- 

 stituted by an ordinary, commonly ciliated, 

 cellular ecderon, which needs no special de- 

 scription. The hard or soft shells which so 

 many of them possess, arise in two modes ; the 

 calcareous and horny integumentary append- 

 ages being, I believe, invariably produced by 

 excretion, while the Ascidian test, which con- 

 tains cellulose, is formed by conversion. It 

 will be advisable to treat of the structure and 

 histological development of these two forms 

 separately ; and, first, of the 



Excretionary integument of the Mollusca. 

 This is to be met with in its simplest form in 

 the Polyzoa,in which the integument (ectocyst 

 of Allman) is formed by a structureless 



membrane containing imbedded calcareous or 

 silicious particles.* 



An admirable example of the calcareous 

 integument formed by excretion is to be found 

 in the shell of Unio and Anodon. The outer 

 surface of the shell in these Lamellibranchs is, 

 as is well known, covered by a brownish or 

 greenish irregular membranous substance, 

 the so called "epidermis" of the shell. This 

 substance, however, by no means constitutes 

 a single membrane ; on the other hand, the 

 surface of the shell is marked by an immense 

 number of closely set, more or less parallel, 

 concentric lines, some of which appear to be 

 formed by rugae of the "epiderm," while others 

 are the free edges of epidermic laminae cropping 

 out under those of older date. Viewing this 

 surface of the shell by transmitted light with 

 a low power, a number of polygonal closely-set 

 areae come into view on depressing the focus 

 through the thickness of the epiderm. 



The inner surface of the shell has, for the 

 greater part of its extent, a pearly or nacreous 

 lustre ; but along the gape of the shell, at a 

 distance of from less than one line, to as much 

 as two or three lines, from the free edge, the 

 nacreous appearance ceases, and we find, 

 instead, a brownish hue similar to that of the 

 epiderm, and becoming gradually more intense 

 till the very margin is constituted by a flexible 

 brown membrane continuous and identical with 

 the epiderm on the exterior. If the surface 

 of the flexible zone be examined as before, its 

 outermost portion appears quite homogeneous; 

 as we pass gradually inwards, however, dots 

 appear in it, and the hard portion of the brown 

 zone presents polygonal areae, precisely resem- 

 bling those under the epiderm on the outer sur- 

 face. Where the nacreous appearance com- 

 mences, these areae disappear, becoming ob- 

 scured by an opaque white substance, which is 

 marked by elevations and depressions, corre- 

 sponding with, though less prominent than, the 

 principal ones upon the external surface. 



If, now, a section perpendicular to the 

 surface and to the concentric lines be taken, 

 and viewed in the same way by reflected light, 

 the cause of the various appearances which 

 have been described will become obvious. 



It will be seen that the thick middle of the 

 shell is composed of three substances; of a 

 very thin external brown layer, the " epi- 

 dermis," and of two other layers more or less 

 equal in thickness ; an external, composed of 

 minute polygonal prisms or columns set per- 

 pendicularly to the surface, and an internal, 

 which looks structureless, with a fracture like 

 loaf sugar. The outer prismatic layer pre- 

 serves its thickness as far as the " brown 

 zone" above described, and then gradually 

 thins out into the flexible marginal membrane. 

 The inner nacreous layer, on the contrary, 

 gradually thins out, and ceases at the com- 

 mencement of the brown zone. The ends of 

 the prisms are, therefore, bare in the brown 



* I am indebted to Mr. Busk, whose extensive 

 researches on these animals are well known, for the 

 information on which this statement is based. 



