2 GIANT FISHES 



number, situated on each side oi the hinder part of the head ; 

 whereas, in the Tarpon these open into a common branchial 

 chamber on each side, protected on the outside by a bony 

 gill-cover or operculum, and with a single opening to the 

 exterior at the back of the head. The Shark has a single nostril 

 on each side of its snout ; the Tarpon (but not quite all Bony 

 Fishes) has two. There is no air-bladder in the Shark, but this 

 organ is developed in the majority of Bony Fishes. The male 

 Shark is provided with curious organs called " claspers ", 

 organs which are unknown in Bony Fishes. Finally, instead 

 of being covered with overlapping scales as in the Tarpon, 

 the body of a Shark is armed with numerous closely-set bony 

 nodules, known as dermal denticles. The claspers, dermal 

 denticles, and the teeth of Selachians are worthy of further 

 consideration. 



Unlike the generality of Bony Fishes, the eggs of a Shark or 

 Ray are fertilized within the body of the female as in most 

 higher animals, and there is consequently a definite union of 

 the sexes. The claspers of the male, which are actually 

 modified portions of the pelvic fins and have a highly compli- 

 cated internal structure, are used for the purpose of introducing 

 the seminal fluid into the body of the female. 



The surface of a Shark or Ray is nearly always rough or 

 prickly to the touch, and this is due to the presence of the 

 innumerable tiny denticles embedded in the skin and covering 

 the whole of the head, body and parts of the fins. These 

 denticles are usually arranged in regular oblique rows, and, 

 examined with a hand lens or under the microscope, each may 

 be seen to consist of a bone-like base, which is embedded 

 under the skin, and therefore invisible during life, and a 

 superficial enamel-covered spine, which projects freely outwards 

 and backwards. An even more intimate examination of one 

 of the denticles reveals that in its structure it is essentially 

 similar to one of the teeth in the jaws, being made up mainly 

 of a substance known as dentine, with an internal pulp cavity 

 and an outer coating of enamel. Thus, the very interesting 

 fact emerges that the teeth of Selachians and Bony Fishes, 

 indeed of all higher animals, including ourselves, must have 

 arisen in the first place from the dermal denticles of the 

 Shark's skin, the denticles in the region of the jaws having 



