INTRODUCTION TO THE COELOMATA 261 



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 withdrawn from the zone of danger.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. 



"The internal anatomy of the lower animals was first studied by 

 physicians and others primarily interested in human anatomy. An un- 

 fortunate consequence is that a large number of names are used in the 

 description of simpler animals which are based on fanciful resemblances 

 between their organs and those of man. As a consequence many of these 

 names are quite misleading. To give some instances : The word stomach 

 in the Lobster denotes part of the stomodaeum, in the vertebrata it sig- 

 nifies part of the entodermic tube. The pharynx ( ) 

 of an earthworm is the stomodeum, in a fish it includes both stomodeum 

 and the first part of the entodermic tube. The term liver has also been 

 much abused. 



"The 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, intes- 

 tine, and rectum. They are applied generally to parts of it succeeding 

 one another in the order above given. The significance of these will be 

 explained in each case : it would perhaps be more logical to sweep away 

 altogether these and a host of similar terms employed to designate other 

 parts of the body, but so deeply are they engrained in zoological litera- 

 ture that such a course would render unintelligible most anatomical de- 

 scriptions of species that we possess." 



