Structure of the Short Sun-Fish. 193 



fish exhibits not a trace of abdominal muscles.* The viscera 

 from the spine to the median line of the belly are inclosed by 

 abdominal walls, consisting of peritoneum on the internal sur- 

 face, of skin on the external, and of a thick layer (4 or 6 

 inches) of the peculiar vesicular tissue already described be- 

 tween 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 con- 

 sequence, 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.-f* This inversion of the 

 muscular masses depends on the stunted condition of the ver- 

 tebral 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 column is still a 

 problem, + and must be sought for, probably, as a circumstance 



* Meckel, Comp. Anatomy, torn. v. p. 185. 



t Meckel, Comp. 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. I have, however, satisfied myself that 

 this mass, although extending forwards to the head, is, in fact, the fin 

 muscle, and that it consists of uninterrupted radiating bundles. The body 

 of this fish, then, contains only six muscles — two for each fin. It appears 

 to swim by a sculling action of the dorsal and anal fins, the tail being a 

 very inefficient organ of locomotion. 



* I havo not assumed the short spinal coid of the orthagoriscus mola as 

 the cause of the stunting of the osseous column, as there are contradictory 



VOL. XXX. NO. I.IX.- — JANUARY 1841. N 



