OF THE SIIORT SUN-FISH. 399 



abdominal walls, consisting of peritoneum on the internal sur- 

 face, of skin on the external, and of a thick layer (4 or G 

 inches) of the peculiar tissue already described between 

 them. This is a conformation exactly corresponding with 

 the embryonic condition of all the vertebrata. The ab- 

 dominal muscles are among the last to be developed, in 

 consequence, in a great measure, of the persistence of the 

 yolk-bag, and the evolution of the abdominal walls from the 

 dorsal towards the ventral aspect. 



The muscles of the spine, also, instead of stretching from 

 head to tail, are reduced to a very small size, and constitute 

 only a weak fan-shaped muscle on each side of the caudal fin. 

 These muscles consist of a small digitation for each of the 

 fin-rays, and appear to me rather to be analogous to the 

 caudal-fin muscles of other fishes than to the great lateral 

 muscles of the spine. 



The thick mass of muscle on each side of the sun-fish 

 consists of the muscles of the anal and dorsal fins ; very weak 

 in other fishes, but developed here in an extraordinary 

 manner — in an inverse ratio to the spinal muscles.* This 

 inversion of the muscular masses depends on the stunted con- 

 dition of the vertebral column, and on the developed state (in 

 regard to form) of the peripheral elements of the skeleton, 

 and is an instance of the dependence of one organic system 

 on another. The morphological cause of the stunting of the 



* Meckel, Comparative Anatomy, torn. v. p. 185, inclines to the opinion 

 that the dorsal portion of the lateral fleshy mass of the sun-fish is a composite 

 muscle, consisting of the anterior part of the usual lateral spinal muscles, 

 and of the muscles of the dorsal fin ; or that the latter had assumed the form 

 and position of the former. 1 have, however, satisfied myself thai this mass, 

 although extending forwards to the head, is in fad the fin-muscle, and that 

 it consists of uninterrupted radiating bundles. The body of tin's fish, then, 

 contains only six muscles— two for each fin. It appears to swim by a scul- 

 ling action of the dorsal and anal lins, the tail being a very inefficient organ 



of locomotion. 



Vol.. I. 2 D 



