262 General Biology 



nerves, thus making it possible for any or all parts of the body to be 

 withdrawn from the zone of danger. 



For toxic injuries, as well as parasitical invasions, which come 

 through the intestinal tract, the student must think of the body, when 

 drawn out completely, as forming a tube within a tube. (Fig. 164.) 



The inner one, called the intestinal, or digestive tract, has an open- 

 ing straight through the body. This means that the inside of the diges- 

 tive tract is really outside the body in so far as exterior environmental 

 conditions may affect it, such as temperature, air, etc. In other words, 

 it is as though one took an ordinary small gas or water pipe and placed 

 it in water. There would be the same kind and quality of water on the 

 inside as there would be on the outside of the pipe. 



The larger outer tube is the outer body wall. 



As the internal anatomy of the lower animals was first studied by 

 physicians and others primarily interested in human anatomy, a large 

 number of names is used in the description of simpler animals which are 

 based on fanciful resemblances between their organs and those of man. 

 Many of these names are, therefore, quite misleading. For example : 

 The word stomach in the lobster denotes part of the stomodeum, while 

 in the vertebrates it signifies part of the entodermic tube. The pharynx 

 ( ) of an earthworm is the stomodeum, while in 



fishes it includes both stomodeum and the first part of the entodermic 

 tube. 



Names taken from the higher animals, which are customarily used 

 in the description of the alimentary canal, are as follows: Mouth or 

 buccal-cavity, pharynx, oesophagus, stomach or crop, gizzard, intestine, 

 and rectum. These names apply to parts succeeding one another in the 

 order above given. Many biologists hold that it would perhaps be more 

 logical- to sweep away altogether these and a host of similar terms em- 

 ployed to designate other parts of the body, but as these terms have 

 become so deeply engrained in zoological literature such a course would 

 render unintelligible most of the anatomical descriptions of species we 

 possess. 



