ir. TEBTEBllATA. 523 



and the alisphenoid. Only behind is there a dorsal element, the 

 snpraoccipital. 



The skull must be roofed in by membrane bones, and of these 

 three pairs are almost constantly present. These are, from behind 

 forwards, a pair of parietals, a pair of frontals, and a pair of nasals. 



Fig. 562.— Sagittal section of liinder part of goat slcull. (From Gegenbaur.) 

 For lettering see fig. 501. 



the latter covering the nasal capsules. Confined to the lower 

 vertebrates is a large membrane bone on the floor of the skull, the 

 parasjDhenoid, which reaches from the basioccipital to the meseth- 

 moid. 



The scheme of the cranium thus outlined undergoes the most modia- 

 catious in the sphenoidal region. Paraspbenoid, on the one hand, and 

 basi-and prespbenoid, on the other, may be substituted for one another, 

 so that when the paraspbenoid is present (fishes, Amphibia) the otliers 

 are small or absent and vice versa (mammals). In the mammals, besides, 

 the alisphenoids fuse with the basisphenoid (greater wings), the orbito- 

 sphenoids with the presphenoid (lesser wings), so there arise here an an- 

 terior and a posterior sphenoid, fused in man to a single sphenoid bone. 

 Mesethmoid and ectethmoids likewise fuse in the mammals to an eth- 

 moid bone. 



The brain case, or cranium, is develoj^ed into the complete skull 

 by the addition of the visceral skeleton, a series of arches which, 

 like ribs, embrace the beginning of the alimentary tract and are 

 related to the cranium, much as arc ribs to the vertebra'. These 

 must be considered as 'p-M'is of the skull, although in part they are 

 shoved backwards and lie under the anterior end of the vertebral 

 column. As the ribs arise in alternation with the musculature 

 {myomeric) , so the visceral arches are similarly related to the gill 

 formation {branch iomeric). Analogous to the cranium the visceral 

 skeleton has a cartilaginous and a bony stage. The visceral skeleton 

 is entirely cartilaginous only in Elasmobranchs, and here it is so 



