The Skeleton of the Fish 49 



behind, being joined by ball-and-socket joints (opisthocoelian) . 

 In most other fishes they are double concave (anxpliccelian) . 

 In sharks the vertebrae are imperfectly ossified, a number of 

 terms, asterospondylous, cyclospondylous, tectospondylous, being 

 applied to the different stages of ossification, these terms referring 

 to the different modes of arrangement of the calcareous material 

 within the vertebra. 



The Intemeurals and Interhaemals. — The vertical fins are 

 connected with the skeletons by bones placed loosely in the 

 flesh and not joined by ligament or suture. Below the dorsal 

 fin (76) lies a series of these bones, dagger-shaped, with the 

 point downward. These are called intemeurals (75) and to 

 these the spines and soft rays of the fin are articulated. 



In like fashion the spines and rays of the anal fin (18) are 

 jointed at base to bones called interhcsmals (77). In certain 

 cases tlie second interhsemal is much enlarged, made hollow and 

 quill-shaped, and in its concave upper end the tip of the air- 

 bladder is received. This structure is seen in the plumefishes 

 {Calamus). These two groups of bones, interneural and inter- 

 hsmal, are sometimes collectively called interspinals. The flat- 

 tened basal bone^f the caudal fin (80) is known as hypural (79). 



The tail of the striped bass, ending in a broad plate which 

 supports the caudal, is said to be homocercal. In more primi- 

 tive forms the tail is turned upward more or less, the fin being 



Fig. 34.— Basal bone of dorsal fin, Holophjchias leptopterus (Agassiz). (After 



Woodward. ) 



largely thrown to its lower side. Such a tail as in the sturgeon 

 is said to be heterocercal. In the isocercal tail of the codfish 

 and its relatives the vertebrae are progressively smaller behind 

 and the hypural plate is obsolete or nearly so, the vertebrae 

 remaining in the line of the axis of the body and dividing the 

 caudal fin equally. The simplest form of tail, called diphycercal, 



'> ',■■ ■ 



, 7.V 



