22 



COMPARATIVE ANATOMY 



the skin and to visceral muscles by way of the myocommata. Dorsal 

 and ventral nerves do not unite. Sympathetic cells and fibers are not 

 segregated to form a sympathetic system. 



Sense organs comparable with those of vertebrates are wanting. 

 A median dorsal pit at the anterior end of the brain is mistakenly spoken 

 of as the olfactory pit. A pigment spot on the brain is likewise somewhat 

 uncritically called the cerebral eye. Amphioxus is, however, very 

 sensitive to light. There is no ear. 



LATERAL LINE 



PERrrONEUM 



Fig. 15. — Squalus acanthias, a typical Vertebrate, in left lateral aspect, with 

 cross sections in three typical regions. The figure illustrates the chief features of 

 vertebrate topographic anatomy. Section A-B is taken through the eyes; section C-D 

 through the spiracles; section E-F through the trunk immediately anterior to the 

 anterior dorsal fin. See also Fig. 465. 



Sub-Phylum Vertebrata (Craniota) 



The vertebrates or craniotes are chordates with a vertebral column 

 and a brain-case. The evolution and perfection of a light and strong 

 endoskeleton has been an important factor in making the vertebrates 

 masters of the world. Exoskeletal structures also appear, as among the 

 invertebrates, but only exceptionally are heavy enough to interfere with 

 the activity of the animal. A many-layered epidermis with various 

 appendages enables the vertebrates to withstand successfully the vicissi- 

 tudes of weather met by land animals. In correlation with their activity, 

 senses multiply and become acute and the brain is much enlarged. The 

 original metamerism characteristic of lower vertebrates becomes much 



