VERTEBRATE ANIMALS 209 



that the tip of the vertebral column turns up at an angle. The 

 caudal fin round this and on the ventral side is expanded into a 

 bifid lobe larger than the dorsal lobe, and with its anterior part 

 larger than the posterior. The result is a markedly asymmetrical 

 structure, and it constitutes what is termed a Heterocercal tail. This 

 is a highly specialised, form of fin derived from a much more primitive 

 type, the Diphycercal fin, such as we find in the tadpole and young 

 dogfish. In these the vertebral column remains straight, and the 

 fin is evenly balanced around it, terminating in a point. In the 

 common bony fish a balanced condition is also to be found, but it is 

 secondarily acquired. The tip of the vertebral column is turned up 

 as in Scyllium, but the simple ventral lobe is prolonged backwards, 

 producing the characteristic " fish tail." This condition is desig- 

 nated Homocercal. There seems little doubt that the median fins 

 are to be regarded as phylogenetically older than the paired fins. 

 Not only do they develop earlier, i.e. are ontogenetically older, but 

 there are forms like Amphioxus and fishes like the Lampreys and 

 Hagfish in which they alone are present and no sign of paired fins 

 are to be found. 



The larger of the paired fins are the pectoral fins, triangular 

 structures arising by their apices from just behind the gills, ventro- 

 laterally about a quarter of the way back. The pelvic fins are 

 smaller, lying in the ventral part of the body and arising not quite 

 half-way back just in front of the cloaca. They differ in the two 

 sexes, and so for the first time in the higher animals we find an 

 external character that enables us readily to distinguish the two 

 sexes. In the female the mesial borders of the pelvic fins pass 

 backwards behind the cloaca parallel with one another and quite 

 free. In the male, however, these two borders unite to form a sort 

 of elongated pouch within which is situated on each side of the middle 

 line a rod-shaped body, the Clasper. These claspers or copulatory 

 organs are grooved on their inner surface and bear at their extremity 

 a group of papillae, on which the scales point in the reverse 

 direction. They are used during copulation, and serve for the con- 

 veyance of the sperms to the oviducts. The paired fins are of great 

 interest, as they are homologous with the fore and hind limbs of 

 higher Craniates, and so with our own arms and legs. Their actual 

 mode of origin is not quite clear, but the most generally accepted 

 account is that they are modified local remnants of a continuous 

 lateral fold running forward from the cloaca to the branchial region ; 

 but we shall return to this point later. 



The fins, median and paired, are all concerned with locomotion 

 in water. The propellant power is obtained from vigorous strokes 

 of the powerful tail with its strong fin, steering is mainly carried out 



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