2o8 COMPARATIVE ANATOMY 



the fins often have a pink tinge. The skin is not covered 

 with scales Hke those of the more familiar bony fishes, but is 

 rough to the touch, and considerable resistance is felt if the 

 finger is drawn from the tail towards the head of the animal. 

 This roughness is due to the presence of innumerable closely 

 set spines or denticles (sometimes called "placoid" scales) 

 imbedded in the cutis, whose shape and structure differs 

 entirely from that of true scales, and most closely resembles 

 that of teeth. In fact, the teeth of the dogfish are nothing 

 more than specialised denticles, and in Acanthias and some 

 other dogfishes, though not in Scyllium, the interior of the 

 mouth is beset with numerous small denticles similar to those 

 of the outer skin. In the thornback ray the denticles are 

 relatively few in number, but large, with sharp prominent spines. 

 The spines of the dorsal fins 'of Acanthias, the long serrated 

 spine on the tail of the sting-ray, the rows of blade-like spines 

 on the rostrum of the saw-fish, are all modifications of skin 

 denticles. Some isolated denticles of Scyllium canicula are 

 shown in fig. 53. Each consists of a flat four-lobed basal 

 plate, bearing a backwardly directed spine on its upper surface. 

 A perforation in the centre of the basal plate leads into a 

 canal called the pulp-cavity, which traverses the centre of the 

 spine, and in life is filled with the so-called pulp, a very vascu- 

 lar form of connective tissue containing large ivory-forming 

 cells or odontoblasts. The bulk of the spine is formed of 

 ivory or dentine, a calcareous tissue traversed by fine ramify- 

 ing canals communicating with the pulp-cavity. Externally 

 the spine is coated with a layer of enamel, a hard structureless 

 substance similar to the enamel of our own teeth. The basal 

 plate is formed of calcified connective tissue, usually described 

 as "cement," though it differs in structure from the cement of 

 our own teeth. A denticle is formed as an upgrowth from the 

 deeper vascular layer of the skin or cutis, covered by the epi- 

 dermis. The cement, pulp, and dentine are formed from the 

 mesoblastic tissue of the cutis, the enamel from the overlying 

 epiblastic epidermis. 



The skin of the dogfish, is closely bound by connective 

 tissue to the muscles beneath, and is difficult to remove. On 

 stripping it away from the heM and anterior part of the trunk, 

 one can see that the trunk muscles, like those of Amphioxus, 

 are divided by connective tissue partitions into a number of 



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