ENDOSKELETON 17 



Among Cetacea scales are known in Neomeris, Phocana and 

 Glohiocephaliis and in the fossil Zeuglodonts which had a more com- 

 plete armor of bony plates. The plates of Neomeris are about 4 mm, 

 across, and are arranged in somewhat regular rows on back and head, 

 on the radial side of the appendages and on the dorsal fin. Their 

 number and distribution are more restricted in the other genera. 



Although closely connected with the skin, the horn cores (p. 195) of Rumi- 

 nants (Bovidae, Cervidae, etc.) are developments of the periosteum of the frontal 

 bones (sometimes of the parietals as well) and are not parts of the dermal 

 skeleton. 



ENDOSKELETON 



The endoskeleton is outlined in cartilage, but membrane bones 

 may be added to the primitive framework, especially in the heads of 

 the higher groups. In Cyclostomes and Elasmobranchs it never 

 passes beyond the cartilage stage, but in all other groups more or 

 less of the cartilage is converted into bone, the amount of this increas- 

 ing, the higher the animal in the scale. To the cartilage bone thus 

 formed, membrane bones are added, these being most numerous in 

 the skull, there being none in vertebral column or ribs and few in the 

 appendicular skeleton. 



AXIAL SKELETON 



This includes the framework of head, trunk and tail, together 

 with a larval structure, the notochord or chorda dorsalis which per- 

 sists largely or wholly in the lower Vertebrates. Skull and vertebral 

 column enclose and protect the central nervous system, the skull, 

 from this standpoint being an enlarged anterior part of the vertebral 

 column; but it must be remembered that Oken's "Vertebral Theory 

 of the Skull" was overthrown years ago. The skull also surrounds a 

 part of the digestive tract and thus a part of it is concerned in taking 

 food. It also embraces organs of special sense which have modi- 

 fied it. The vertebral column, made up of separate metameric 

 elements, the vertebrae, gives strength to the body and serves for the 

 attachment of trunk and caudal muscles, while the ribs and sternum, 

 when present, enclose the greater part of the viscera. Vertebrae, 

 sternum and ribs either persist as cartilage through Kfe, or, preformed 

 in cartilage, ossify later to varying extents. The skull has a cartilage 

 framework, the chondrocranium, most of which (Cyclostomes, 



