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AN INTRODUCTION TO ZOOLOGY 



narrows off considerably, but always remains open, and the whole 

 structure sinks down into the dermis, below the level of the epi- 

 dermis, causing the enamel epithelium and the adjacent cells of the 

 Malpighian layer to become infolded in a very characterictic manner. 

 The main part of the papilla remains to form the pulp, which with 

 the odontoblasts and dentine of the blade and entire basal plate, by 

 far the largest part of the denticle, is the product of the dermis. 

 The enamel alone, at any rate the greater part of it, comes from 

 the epidermis, and, according to some authorities, this also receives 

 contributions from the underlying dentine. 



The second variety of exoskeletal element in the dogfish takes 

 the form of fairly long cartilaginous rods or rays supporting the 

 distal parts of all the fins. In consequence of their being 

 developed in the dermis they are termed dermal fin rays or 

 dermotrichia. 



Muscular System. 



The muscles of the dogfish, like those of the frog, are pf 

 two varieties, striate and non-striate. The latter, including also the 



Ms M 



FIG. 68. Sketch to show arrangement of myotomes in Scy Ilium, 



E., epiaxial myotomes ; H., hypaxial myotomes ; L., lateral line ; M., myotome ; 

 Ms., myosepta. 



cardiac muscles, are confined to the various viscera. If the skin be 

 stripped off the body muscles will be seen beneath it arranged in a 

 very characteristic manner. Unlike the frog, the fibres are not 

 bound together to form definite discrete muscles, save in certain 

 parts, but arranged in narrow columnar sheets. These extend from 

 the lateral line dorsalwards and ventralwards in parallel zigzag 

 strips known as muscle segments or myotomes, which meet in the 

 mid-dorsal and mid-ventral line and alternate with the vertebrae, a 



