THE SKELETON IN FISHES. THE FINS. 115 



The tail of Amia is remarkable from the fact that as a 

 rule to each neuromere, as determined by the exit of the 

 spinal nerves, there are two centra, a posterior one which 

 bears nothing, and an anterior one which bears the neural and 

 haemal arches, these being throughout the vertebral column 

 connected with the centra by cartilaginous discs. 



In most TELEOSTEANS but not in the Plectognathi the neural 

 arches are continuous with the centra, which are nearly always 

 deeply biconcave. 



In some cases many of the anterior vertebrae are anky- 

 losed together and to the skull. The vertebrae often articu- 

 late with one another by means of obliquely placed flattened 

 surfaces, the zygapophyses. The centrum in early stages of 

 development is partially cartilaginous, but the neural arches 

 and spines in the trunk at any rate, pass directly from the 

 membranous to the osseous condition. 



FINS. 



The most primitive fins are undoubtedly the unpaired ones, 

 which probably originally arose as ridges or folds of skin 

 along the mid-dorsal line of the body, and passed thence 

 round the posterior end on to the ventral surface, partially 

 corresponding in position and function to the keel of a ship. 



In long 'fish' which pass through the water with an 

 undulating motion such simple continuous fins may be the 

 only ones found, as in Myxine. To support these median fins 

 skeletal structures came to be developed ; these show two 

 very distinct forms, viz. cartilaginous endoskeletal pieces, the 

 radialia, and horny exoskeletal fibres, the Jin-rays. Me- 

 chanical reasons caused the fin to become concentrated at 

 certain points and reduced at intervening regions. Thus a 

 terminal caudal fin arose and became the chief organ of 

 propulsion, and the dorsal and ventral fins became specialised 

 to act as balancing organs. 



In some of the earlier Elasmobranchs, the Pleuracanthidae, 



82 



