VERTEBRATE ANIMALS 225 



septa between the gill slits, which segment first into two and then 

 into four pieces on each side. The basihyal and basibranchial are 

 separately developed. 



Appendicular Skeleton. 



The remaining part of the skeleton consists of the supports 

 of the median fins, the paired fins and the girdles supporting the 

 latter. 



The dorsal fin is composed of two morphologically distinct parts ; 

 firstly, endoskeletal structures known as pterygiophores or somactidia , 

 and secondly, exoskeletal structures, the dermotrichia. The somac- 

 tidia take the form of a series of cartilaginous rods (12 in the first 

 dorsal fin), attached to a corresponding number of neural spines. 

 Each rod consists of a short basal element, these tend to fuse to form 

 basalia, and a distal longer piece, the radialium, bearing at its 

 extremity two polygonal plates. To these plates are attached a 

 number of dermotrichia, which are horny fibres lying in the dermis 

 and constituting the support of the main part of the blade of the fin. 

 The ventral fin is somewhat similar in build, but the cartilaginous 

 elements are reduced, while in the caudal fin they disappear alto- 

 gether, leaving the dermotrichia to be borne directly by the neural 

 and haemal spines which are greatly enlarged in this region. 



The pectoral girdle consists of a strong hoop of cartilage, 

 irregular in shape and incomplete dorsally, lying embedded in the 

 muscles of the body wall a short distance behind the last gill arch. 

 The median part forms a fairly straight bar running transversely and 

 expanded slightly in the centre to form a thinner plate which passes 

 forward a little to form a platform supporting the hinder end of the 

 floor of the pericardial cavity. This may be termed the coracoid 

 portion of the girdle, and in reality it consists of two bars, as is 

 clearly shown in its development, corresponding with the coracoids 

 of Rana, but they have met in the middle line without the inter- 

 vention of a sternum and fused so as to lose their individuality. 

 The laterial extremity of the bar is marked on each side by three 

 smooth articular surfaces on its posterior aspect which bear the 

 proximal cartilages of the pectoral fin. The remaining parts of the 

 girdle, the scapular portions, are horn-shaped, curved tapering rods 

 passing dorso-mesiaLly to end in the muscles on each side of the 

 vertebral column. 



The basalia of the pectoral fin, as has been noted, are three in 

 number, representing the fusion of a series of somactidia. They 

 increase markedly in size from before backwards, and are known 

 as the pro-, meso- and meta-pterygium respectively. Following 

 these are a series of about 16 radiala, which again terminate in 



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