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AN INTRODUCTION TO ZOOLOGY 



cells become more and more flattened. Scattered among them here 

 and there are spherical glandular or mucous cells, which are not 

 aggregated to form glands as in the frog. The dermis is composed 

 of two layers, an outer of connective tissue, and a lower thicker layer 

 of tough fibrous tissue. It contains blood-vessels and nerves and a 

 number of pigment cells arranged in groups to form the well-marked 

 spots. The skin is rough to the touch, particularly if the hand be 

 drawn from behind forwards, owing to the presence in it of small 

 close-set scales with projecting points directed backwards and 





FIG. 67. Denticles of Scyllium. 



A., surface view of denticle ; B., side view of denticle : I. IV., successive stages in the 

 development of a denticle. 



C.B., formative cells of basal plate ; D., dermis; d., dentine ; D.P., dermal papilla ; E., epi- 

 dermis ; e., enamel ; e.e., enamel epithelium ; M., Malpighian layer ; P., pulp ; P.C., pulp cavity ; 

 Pi., pigment cell. 



forming a characteristic exoskeleton. When dried the skin is known 

 commercially as shagreen, and is utilised for polishing purposes. 

 These scales are of a peculiar type known as placoid scales or dermal 

 denticles, and in their production both epidermis and dermis take 

 part. They differ completely from the scales of the ordinary bony 

 fish, which are entirely dermal structures. 



A denticle consists of a quadrangular basal plate and a blade or 

 spine set at an acute angle to this. The basal plate lies firmly 



