8 VERTEBRATE ANIMALS OF THE UNITED STATES 



only for grasping but for chewing their food, the teeth are firmly set in 

 alveoli — deep cavities in the jaw bone — and have a variety of forms 

 which fit them for grasping, cutting, gnawing or chewing. The teeth 

 of alligators and the extinct toothed birds occupy a mid-way position, 

 being elongated cones in form and set in alveoU. 



The digestive canal in vertebrates is characterized in general by 

 great length and by the presence of two digestive glands of large 

 size, the liver and pancreas, which originate as outgrowths of it, also 

 by the mesenteries in the body cavity which support its various divi- 

 sions. The intestine opens at the hinder end either directly to the 

 outside through the anus, or into a cloaca which receives also the dis- 

 charges of the excretory and reproductive organs. 



The excretory and reproductive systems are closely associated in 

 vertebrates, often possessing ducts in common through which their 

 products find a way to the outside. Reproduction is in all cases 

 sexual; with a few exceptions among fishes the sexes are separate; sexual 

 dimorphism is universal. 



The central nevvous system of vertebrates, the spinal chord and 

 brain, has no homologue among invertebrates. It originates in the 

 embryo as a middorsal ectodermal groove, which sinks beneath the sur- 

 face and forms a tube. The anterior portion of this tube widens and 

 forms a series of five vesicles the walls of which become thick and 

 folded, and the brain comes into existence; the posterior portion becomes 

 the spinal chord. The cranial and spinal nerves proceeding from the 

 brain and spinal chord to the special sense organs and the muscles 

 of the trunk and appendages have a metameric arrangement; the same 

 is true also of the sympathetic nervous system, which is intimately 

 joined with them and controls the activities of many of the viscera. 



The characteristic special senses of vertebrates are smell, taste, 

 sight and hearing. The sensory perceptions of aquatic vertebrates, 

 especially fishes, are in many respects sharply different from those of 

 animals which live in so attenuated a medium as the air, vibrations of 

 the water being the most important stimulating agents w^hich cause 

 their sensory reactions. Vibrations of very low frequency, such as 

 are produced by impacts on the water, currents and surface waves, 

 undoubtedly affect the skin over the whole surface of the fishes body; 

 the lateral line, which is a speciahzed portion of the skin, is sensitive 

 to vibrations of higher frequency, such as shght oscillations of the 

 whole mass of the water, which are, however, too sHght to stimulate the 

 skin itself; the ear, which is a modified portion of the lateral line, is 

 attuned to vibrations of still higher frequency, such as cause the sensa- 



