VERTEBRATA. 17 



pectoral fins are larger and more uniform in their existence than 

 the posterior; they also commence their development at an earlier 

 period; in all of which they resemble the embryo of the higher 

 classes. The ventral fins are wanting in the apodes, as well a*s in 

 several species of other orders ; whilst the pectoral fins are generally 

 present. 



The anterior fins are generally joined to the back of the skull by- 

 means of an osseous belt, formed behind by the scapula, and in front 

 by the coracoid bones. A humerus, generally long and angular, is 

 attached to these above, and to the radius and'ulna below. To these 

 succeed the carpal bones, and the member is terminated by long 

 and numerous phalanges. The posterior members, or ventral fins, 

 have no connection to the spine, but are suspended from the rib-like 

 iliac bones, and placed on the lower part of the trunk, at a variable 

 distance between the head and anus. The presence of a sacrum 

 in this class would be injurious to the free motion required in 

 swimming. In the abdominal fishes the pelvic bones are uncon- 

 nected to the skeleton : and in the apodal families they are wholly 

 absent. The long phalanges of the feet are attached directly to the 

 pelvic bones, there being rarely a trace of intermediate bones 

 developed. 



The want of symmetry observed in the skeletons of many fishes 

 forms a singular peculiarity in this class. In the pleuronectes, for 

 instance, one side is turned upwards, instead of the back ; both 

 eyes are placed on the same side; the cranial vertebra- seem 

 twisted in their long axis ; and the lower part of the head is imper- 

 fectly developed. The bones, especially the intermaxillary, are 

 much larger on the side opposite to that on which the eyes are 

 placed. 



RECAPITULATION. 



1. The bones of fishes resemble the embryotic condition of the 

 osseous system in the higher classes of animals. 



2. The spine is the most important part of their skeleton. 



3. The spinal column enjoys very (ree motion, chiefly in the 

 lateral direction. 



4. The construction of the head is as simple in the cartilaginous 

 as it is complex in the osseous fishes. 



5. The os hyoicles is greatly developed to support the branchial 

 apparatus. 



6. The anterior fins are larger, earlier developed, and more uni- 

 form in their existence than the posterior. 



7. The pelvic bones and posterior fins are absent from the ske- 

 letons of many cartilaginous fishes. 



8. Want of symmetry is a striking peculiarity in the skeletons 

 of many fishes. 



' * evers 2 



