Skeleton of Common Perch. 43 



is given to the organ of motion thus constituted. The imbrication 

 of the spinous first dorsal fin shows that the movements of the 

 vertebral column must be but limited in the vertical plane, and 

 whilst for rising and sinking leisurely, the expansion and con- 

 traction of the air-bladder may suffice, the animal is seen to be, 

 as experiments have shown it is, dependent upon its pectoral and 

 ventral fins for giving to the movement generated by the lateral 

 strokes of its tail an upward or downward direction. Besides thus 

 directing movement, the ventral fins serve by extension to maintain 

 the fish at any desired level in the water; and the pectorals, by 

 moving horizontally, are competent to move the animal, though 

 but feebly, in a forward or backward direction. The anterior dorsal 

 fin has a great vertical as well as anteroposterior development 

 which, besides subserving the purpose of defence for a fish with 

 a dentition ' en fin velours' serves to maintain the fish in a position 

 of stable equilibrium with which the large size at once of the 

 muscles along the upper dorsal region and of the air-bladder at 

 a lower level below them, would otherwise have been incompatible. 

 The tail-fin, having its upper and lower lobes equally developed, 

 does not retain any indication of the more generalized heterocereal 

 form of tail which many Malacopteri, such as the herring and 

 salmon, and some Acanthopteri show. The plane of greatest trans- 

 verse sectional area lies at a point about one-fourth the entire 

 length of the animaFs body from the tip of its snout; the 

 narrowing from this plane forwards to the tip of the snout is, in 

 a well-fed fish, sudden and abrupt. The absence of both neck 

 and sacrum, which is otherwise expressed by saying that fish 

 possess only trunk and tail vertebrae, is as obviously correlated 

 with the peculiarities of these animals^ locomotive, as the absence 

 of a sternum is with the peculiarities of their generative functions. 



The suspensorium is articulated moveably to the outer and back 

 part of the cranium by a glenoid cavity which is formed for its 

 head by the squamosal, opisthotic, and prootic bones. Into the 

 composition of the suspensorium there enter four bones, viz. the 

 hyomandibular which forms the articular head just mentioned, 

 the symplectic, the quadrate, and the praeoperculum. The lower 

 end of the quadrate is received into the articular cavity formed by 

 the OS articulare of the lower jaw. On the inner surface of the 

 suspensorium the stylohyal comes into relation with the synchon- 



