SKELETON. 



531 



Appendages. The pectoral fins are attached to the 

 shoulder-girdle just behind the branchial aperture. The 

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

 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 effective swimming 

 organ is 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. Each cen- 

 trum in the trunk region bears superior neural 

 processes, uniting in a neural arch crowned 

 by a neural spine, and transverse processes 

 projecting from each side. Articulated to 

 the distal ends of the transverse processes 

 are the downward curving ribs, and also more 

 delicate intermuscular bones which curve 

 upwards. In the caudal vertebrae (Fig. 259), 

 the centra (c.) bear not only superior neural 

 processes (n.a.), but also inferior haemal pro- 

 cesses (A. a.) ; they are of course without ribs. 



At the end of the vertebral column lies 



fan-shaped hypural bone which helps to 



a 



FIG. 259. Cau- 

 dal vertebra 

 of haddock. 



n.a., Neural arch ; 

 c. , centrum ; 

 //.a., hcemal 

 arch. 



support the tail, and is developed from an 



enlarged haemal arch. The fin-rays are 



jointed flexible rods, which in the dorsal 



and anal fins are attached to the ends of 



interspinous bones alternating with the neural and haemal 



spines, and connected with them by fibrous tissue. 



The skull includes the following bones, which may be 

 grouped in the following regions (the membrane bones in 

 italics) :- 



(a) Around the foramen magnum : basi-occipital, two ex-occipitals, 



and a ^/ra-occipital. 



(b] Along the roof : supra-occipital, parietah, f rentals, mesethmoid, 



nasals. Beneath the parietah lie the alisphenoids. 



