THE ENDOSKELETON 145 



become enclosed by cartilaginous capsules, differing some- 

 what in their development, according to the needs of the organ. 

 Thus the nasal capsules remain open anteriorly for the free 

 admission of the fluids to be tested, the eye-capsules involve 

 the sclera alone, while the otic capsules usually become entirely 

 closed and develop fairly thick walls, since sound vibrations can 

 pass easily through solids and do not need a special opening. 

 As these several cartilaginous elements, the para- and prae- 

 chordals and the sense capsules, increase in size, they fuse 

 together in about the following manner. The two trabeculse 

 expand anteriorly and fuse with each other across the middle 

 line and with the nasal capsules as well ; extending backwards, 

 they fuse with the parachordals. These latter, growing in 

 width, fuse both with the otic capsules and with each other, 

 including in this fusion the anterior end of the notochord, 

 which becomes lost in the general mass. 



There is thus formed a single, curiously shaped piece of 

 continuous cartilage, composed of all the elementary pieces, 

 with the natural exception of the otic capsules, which must 

 remain free to allow the turning of the eyeball (Fig. 38, C). 

 These pieces fuse so completely that all boundaries are lost, 

 and we can speak only of a parachordal or a trabecular re- 

 gion, and so on, without assigning definite boundaries. This 

 consolidated piece is termed the primordial skull or chondro- 

 cranium, and remains at this stage in selachians, where it is 

 characteristic of the entire Order, being throughout life with- 

 out trace of ossification and with such slight modifications 

 only as are necessary for the adaptations of the various adult 

 forms. It is a natural supposition drawn from common ex- 

 perience that a skull is intended for the protection of the 

 brain, but in this case the function is rather that of support, 

 since it lies laterally to and in part beneath the brain, leaving 

 practically the entire dorsal and the anterior part of the ven- 

 tral aspects without protection. In the adult selachians, in- 

 deed, these deficiencies are made up in part by the formation 

 of firm membranes, continuous with the cartilage and closing 

 in the open fontanelles, but they are plainly secondary modifi- 



