STRUCTURE OF HADDOCK 617 



apertures — named in order from before backwards. Along 

 the sides of the body runs the dark lateral line containing 

 sensory cells. There are three dorsal and two anal fins, 

 and an apparently symmetrical tail fin. 



Skin. — The small scales are developed in the dermis, 

 and consist of flexible structureless bone (vitrodentin). 

 Their free margin is even, a characteristic to which the 

 term cycloid is applied, in contrast to ctenoid, which 

 describes those scales which have a notched or comb-like 

 free margin. Over the scales extends a delicate partially 

 pigmented epidermis. 



Appendages. — The pectoral fins are attached to the 



Fig. 351. — External characters of a Teleostean — 

 a carp [Cyprinus carpio). — After Leunis. 



R., Dorsal unpaired fin; S., homocercal caudal fin; A., anal fin; 

 B., B., pectoral and pelvic paired fins. Note also the lateral 

 line and barbule. 



shoulder-girdle just behind the branchial aperture. The 

 pelvic or ventral fins, attached to what is at most a rudi- 

 ment of the pelvic girdle, lie below and slightly in front of 

 the pectorals— far from the normal position of hind-limbs. 



Muscular system. — The main muscles of the body are 

 disposed in segments — myotomes or myomeres, separated 

 by partitions of connective tissue. The efl^ective swimming 

 organ is the posterior body and the tail, as contrasted with 

 the pectoral fins in the skate. 



Skeleton. — The vertebral column consists of biconcave 

 or amphiccelous bony vertebrae, and is divided into two 

 regions only, caudal and pre-caudal. The spaces between 

 the vertebrae are filled by the remains of the notochord. 



