724 COMPARATIVE ANATOMY 



It can, therefore, easily be seen that any conditions such as dust and 

 moisture that may be in the atmosphere surrounding the outside of this 

 pipe will quite likely be found on the inside. 



During the development of the nervous system there is a connection 

 between the lumen of the neural tube and the gastrular mouth so that 

 there is a temporary connection between the neural tube and the gas- 

 trocoele. This connection is called the neurenteric canal (Fig. 328, B, 

 ne.c). This connection, however, soon disappears so that the gastrocoele 

 is a closed sac with no opening whatever to the outside of the body until 

 the mouth and anal openings are pushed in from the ectoderm as already 

 mentioned. 



Definite names are given to the various structures of the growing 

 embryo. The mouth opening is called the stomatodeum ( ). 



The mid portion connecting the mouth with the anal opening is called 

 the mesodeum ( ), and the caudal part, which like 



the stomatodeum is ectodermic, is known as the proctodeum. 



This does not mean that in every animal in which an ectodermal 

 mouth and anus has developed that the ectodermal structures take up 

 the same length of the digestive system. In the articulates (crustaceans, 

 insects, and spiders) the stomatodeum and proctodeum are much longer 

 and larger proportionately to the mesodeum than in the higher forms of 

 life, in fact, in the vertebrates, the digestive canal is mainly mesodeal and 

 therefore endodermic. The mouth and anal regions composed of ecto- 

 derm are but a small portion of the entire digestive system. 



The jaws, teeth, and tongue, which will be taken up separately, do 

 not develop from the simple digestive tube which has just been described, 

 but the other parts of the digestive system, even the most complicated 

 ones, have come from this tube alone by a growing in length, by enlarge- 

 ments of various kinds, by foldings, by outpushings and inpushings. Not 

 only have such complex organs as the liver and spleen, thyroid and 

 thymus glands, as well as many others, come from this endodermal tube, 

 but the entire breathing apparatus of chordates has arisen from its 

 cephalic end. 



As some chordates, such as fishes, live in water, they require a to- 

 tally different type of breathing mechanism than those which live on 

 land, yet their branchial or gill system and the land living pulmonary 

 or lung system have in each case developed from the same simple diges- 

 tive tube. It must be remembered that this is only true of chordates. 

 Animals not chordates do not show such close relationship between the 

 digestive and respiratory systems. 



The intestinal tract, if cut in cross section and examined microscop- 

 ically will be found to be composed of four layers of different types of 



