VII MOLLUSGA— INTEGUMENT, MANTLE, VISCERAL DOME 53 



In such animals the surrounding water can circulate through the usually open 

 mantle cleft and the mantle cavity. AVe here find protuberances, papillfe, tentacles, 

 etc., carrying sensory organs, all along the free edges of the mantle, whereas, in 

 bivalves which inhabit mud or bore into wood, rook, etc., such organs are mostly 

 found massed together round the edges of the branchial and anal apertures. 



The Edge of the Mantle. 



The edge of the mantle often forms a number of diverging folds, which in trans- 

 verse section look like finger-shaped processes. The outermost fold is always applied 

 to the shell. The edge of the mantle may also be beset with one or more rows of pro- 

 tuberances, papillffi, or tentacles, and often contains unicellular or multicellular 

 glands, mucous glands, and others which have been considered to be poison glands for 

 protective purposes. Tactile sensory cells are very common. Eyes are rarely developed 

 on the edge (</. section on the Sensory Organs). 



In the Pectiiiidce, Spuiiifi/Iidcc, and Limidcc, the inner fold of the mantle has a 

 somewhat broad border, which, when the shell is open, projects from the mantle 

 towards the median line of the body (Fig. 23, p. 16). The free opposite edges of 

 these folds (flaps, or curtains), springing from right and left, may meet in such a 

 way as to shut off the central part of the mantle cavity even while the shell is open, 

 apertures only remaining anteriorly and posteriorly. 



F. Cephalopoda. 



Integument. 



The integument of the Cephalopoda consists of an outer cylindrical 

 epithelium, and a subjacent cutis in the form of thick connective tissue. 

 In this cutis, not far removed from the epidermis, and above a layer 

 of connective tissue plates (which are refractive and often shimmer like 

 silver), there are large pigment cells or ehpomatophores which, by their 

 alternate contraction and expansion, bring about the well-known changes 

 of colour in these animals. 



These chromatophores are single cells containing yellow, brown, black, violet or 

 carmine pigment, either as fluid or in small granules. The layers containing them 

 are either single or double ; in the latter case, the colour of the pigment in the one 

 layer of chromatophores diff'ers from that of the chromatophores in the other. 

 Radial fibres, arising from the suiTounding connective tissue, are attached to each 

 ehromatophore, round that equator which lies parallel to the integument. The 

 chromatophores are enveloped in a special, possibly elastic, membrane, and when 

 contracted are almost globular ; the pigment corpuscles are then crowded together. 

 The chromatophores expand equatorially, diminishing the distance between their 

 poles, i.e. they become much flattened. In this condition, they may, further, throw 

 out fine branches, the pigment gi-annles being thus spread out over a large surface. 

 It was formerly believed that the expansion of the chromatophores was caused by the 

 contraction of the radial fibres, which were thought to be muscular, but more recent 

 investigations have shown the fibres to be of the nature of connective tissue. The 

 changes of colour, which are of great physiological and biological interest, and which 

 are partially under the control of the animal, are brought about by the alternate 

 contraction and expansion of these variously coloured chromatophores 



