vii MOLLUSCA INTEGUMENT, MANTLE, VISCERAL DOME 39 



III. The Integument, the Mantle, and the Visceral Dome. 



The whole body is covered by a single layer of epithelium, which, 

 in parts not protected by the q 



shell, may be more or less 

 ciliated. This layer is very 

 rich in glands which are 

 almost exclusively unicellular ; 

 some of these lie in the epi- 

 thelium itself, while some 

 have sunk into the subjacent 

 tissue, their ducts, however, 

 passing between the epithelial 

 cells. 3 



The layer immediately 

 beneath this body epithelium 

 is called the corium, and con- 

 sists of connective tissue and 

 muscle fibres. It is, how- 

 ever, not distinctly marked 

 off from the tissues beneath 



jj. FIG. 4!). Section of the integument of Daudebardia 



, . . , rufa (after Plate). 1, Epithelium ; 2, 3, 9, various forms 



Ihe pigment IS almost O f unicellular glands; 4, globular pigment cells; 5, 7, 



always found in the Cells of unpigmented cells of the connective tissue ; 6, muscle 



the subepithelial connective ; 8 branche aml anastomosin s cells of the con - 

 tissue. 



nective tissue containing pigment. 



A. Placophora. (Of. the sketch of the Outer Organisation, p. 29.) 



The Chiton is provided dorsally with eight consecutive shell-plates (Fig. 1, p. 2), 

 which overlap in such a manner that the posterior edge of each plate covers the anterior 

 edgeof the next. These plates are bilaminar. The outerand upper layerwhich forms the 

 dorsal surface is called the tegmentum, the lower hidden layer the articulamentum. 

 As a rule, the tegmentum of the anterior plate only is as large as the articulamentum 

 beneath it ; in the other plates, the latter is the larger and projects beyond the 

 former laterally and anteriorly. These projecting parts of the articulamentum, 

 called apophyses, slide under the plate next in order anteriorly. Between these 

 two layers, tissue is found, which is a continuation of the dorsal integument. 

 The tegmentum is penetrated by canals of various sizes, which open at its 

 surface through characteristically arranged pores. 1 The tegmentum consists of a 

 horny or chitinous substance, which may be considered as a cuticular formation, 

 impregnated with calcareous salts. The articulamentum is compact and free from 

 canals ; it contains little organic substance, and much calcareous salt. It alone 

 answers to the shell of other Molluscs, while the tegmentum must be considered as 

 a calcified cuticle covering the true shells (the articulamenta) as a continuation of 

 the cuticle of the zone which encircles the eight shell-plates. This zone carries 



1 On the relation of these canals and pores to peculiar sensory organs and eyes on 

 the shell of the Chiton, cf. section on Sensory Organs, p. 166. 



