ADAPTATION OF THE FISIl'S SKULL, ETC. 57 



soft rajs. A, the anal fin, has 3 spinous + 8 soft rays. 

 C, the caudal fin, has 18 rays. 



When the piscine modification of the vertebrate skele- 

 ton is contemplated in relation to the life and movements 

 of a fish in its native element, every departure from the 

 archetype is seen to be in direct relation to the habits 

 and well-being of the species. 



The large head has been compared to the embryonic 

 disproportion of that part in higher vertebrates ; but the 

 head of a fish should be of the size and shape best fitted 

 to overcome the resistance of water, and to facilitate rapid 

 progression through that element ; the head must, there- 

 fore, grow with the growth of the body. Accordingly, 

 the large skull-bones always show the radiating bony 

 filaments in their clear circumference, which is the seat 

 of growth ; and hence the number of overlapping squa- 

 mous sutures which least oppose the progressive exten- 

 sion of the bones. The cranial cavity expands with the 

 expansion of the skull, but the brain undergoes no cor- 

 responding increase ; it lies at the bottom of its capacious 

 chamber, which is principally occupied by a loose cellular 

 tissue, situated, like the "arachnoid" membrane in man, 

 between the brain-tunics, called " pia mater" and " dura 

 mater," and having its cells filled by a light, oily fluid ; 

 thus the head is rendered specifically lighter than if 

 growth only, and not the modelling absorption also, had 

 gone on. The loose connection of the haemal arches and 

 their parts, including most of what are called " bones of 

 the face," seems like the retention of a condition observ- 

 able in the partially-developed skull of the embryos of 

 higher animals ; but this condition is subservient to the 

 peculiar and extensive movements of the jaws, and of the 

 bony supports of the breathing machinery. Not any of 



