200 



BONY FISHES 



vii. 5- 



radials the fins are also supported by a more superficial set of bony- 

 rods, the dermal fin rays (dermotrichia or lepidotrichia), which may 

 be considered as modified scales and accordingly lie superficial to the 

 radials. These dermal fin rays are usually forked at their tips. They 

 make an extra support for the fin margin and to them are attached the 

 muscles that serve to throw the fin into folds. 



In the tail region the internal skeleton is not quite symmetrical and 

 shows signs of origin from an animal with a heterocercal tail. The 



<ev.msx. 



3<j-mand. 



Fig. 121. Muscles of a teleostean fish, mainly based on Mullus. 

 ad-mand. adductor mandibulae; ep. epaxonic muscles; h. horizontal myoseptum; hy. hypo- 

 branchial muscles; hyp. hypaxonic muscles; lev. max. levator maxillae; my. myocomma; 

 op. operculum. (From Ihle.) 



notochord turns up sharply at the tip, so that the neural spines are 

 very much shorter than the haemal spines, known here as hippural 

 bones. The final portion of the notochord is often surrounded by a 

 single ossification, the urostyle, and the whole makes a rigid support 

 for the dermotrichia of the tail. Such a tail with internal asymmetry 

 but external symmetry is said to be homocercal. 



The myotomes are arranged in a complicated pattern having the 

 effect that contraction of each affects a considerable section of the 

 body (Fig. 121); in fast swimmers such as the tunny each myotome 

 may overlap as many as nineteen vertebrae. Between the lateral and 

 ventral muscle masses there is in many fishes a layer of red muscle 

 and this is especially well developed in the tunnies and bonitos. 



The paired fins are similarly supported by ossified radials, covered 

 by dermal fin rays. At the base the radials are connected with 'girdles' 

 lying in the body wall. The pectoral girdle (Fig. 118) consists of a 

 cartilaginous endo-skeletal portion in which ossify the scapula, cora- 

 coid, and sometimes mesocoracoid, while dermal bones, large cleith- 

 rum, and one or more small clavicles, become attached superficially. 



