328 COMPARATIVE ANATOMY. 



is tlie reason wliy these separate arms were formerly regarded as 

 parasitic organisms (Hectocotylus) , 



Steei\strup, J. J., Hectocotyldannelsen. Kongl. Dansk. Vid. Selsk. Skrifter. V. E. 

 4Bd, 



Integument. 

 § 255. 



The body of the Mollusca is covered by a soft dermal layer, 

 which isj as a rule, so closely interwoven with the subjacent muscles 

 as to form a kind of dermo-muscular tube, just as in the Vermes. 

 The locomotor organs are formed by the great development of the 

 musculature in certain regions of the body, and by the consequent 

 differentiation of some parts of the dermo-muscular tube. 



Inmost divisions of the Mollusca there is an investment of 

 cilia during the larval stages, which later on extends over the whole, 

 or some parts of the body. The cilia in the velar circlet (§ 248) are 

 those that are most markedly developed. The rest are chiefly 

 found in the respiratory organs. In the Cephalopoda even, nearly 

 the whole surface of the germinal disc (save the gills) is covered with 

 cilia during development; later on a ciliated epithelium may be 

 found on the yolk-sac also. 



The integument is easily distinguishable into an epidermis and 

 cutis. In many Heteropoda (Carinaria, Pterotrachea) the latter is 

 specially modified ; a strong transparent layer of connective tissue 

 preventing any great amount of change in the form of the body. In 

 the rest of the Mollusca the body is generally prevented by the 

 shell, which is developed from the integument, from undergoing any 

 great changes in form. 



The coloration of the body is due to the deposits of pigment in 

 the integument. The most remarkable structures concerned in 

 coloration are those which are found in many Pteropoda, and in all 

 Cephalopoda — the " chromatophores.'' They are rounded cells, 

 which are placed at various depths in the integument; they are 

 filled with granular pigment, and are provided with radial muscular 

 fibres at their periphery ; when these fibres contract the cell broadens 

 out, and the pigmented contents are thereby distributed in such a 

 way that they become easily visible to the eye, as large, stellate, 

 and often branched spots. Plate-like elements are found deposited 

 in a layer, which is sometimes differentiated, and these give a 

 silvery appearance to many parts of the body (spangled layer). The 

 varying character of these several layers produces tJbat play of colour 

 which we admire in the skin of the living Cuttlefish. 



Other deposits are found in the integument, such as those formed 

 of carbonate of calcium, which are common in tlse Gastropoda ; these 

 are either simple granules, or larger rounded concretions, or they 

 may be rod-shaped, denticulate, or even branched ; there is often a 



