CHAPTER XVIII. 



INTRODUCTION TO THE COELOMATA 



From what has already been learned it is known that animals may 

 be divided, according to whether or not they have a backbone, into two 

 great groups the vertebrates and the invertebrates. Also, according to 

 whether they are composed of one or more cells into protozoa and meta- 

 zoa, and the latter according to the number of germ layers each form 

 develops, into diploblastic and triploblastic organisms. 



Now we come to another common method of classifying them into 

 two groups the coelomata and the acoelomata. 



With the exception of the frog, all of the animals studied so far 

 the protozoa and coelenterata belong to the acoelomata, because they 

 have no additional cavity between the digestive tract and the body wall. 

 Coelomata have such a body-cavity. All animals higher in the scale of 

 life than hydra are coelomates. 



It will be remembered that in hydra there was a thick mucilaginous 

 substance the mesoglea formed between the ectoderm and entoderm. 

 In some of the lower forms of acoelomata there are processes stretching 

 across from inner to outer germ-layer, which often secrete fibers which 

 become "connective tissue or may be developed into muscular fibers. 

 Where the cells and fibers are sparse, this space is said to be a primary 

 body-cavity. Where they are abundant, it is called parenchyma 

 ( ) or connective tissue." 



This body cavity, also known as the coelomic cavity (Fig. 162) or 

 coelom (Gr. koiloma=a thing hollowed out), consists of "one or more 

 pairs of sacs with perfectly defined walls lying at the sides of the ento- 

 dermic tube. In the adult these sacs join each other above and below 

 the entoderm, and the adjacent walls entirely or partly break down, and 

 thus one continuous cavity results. The wall of the coelom and the tis- 

 sues derived from it are known as mesoderm." 



The distinctive difference between the primary body cavity of the 

 coelenterates and this secondary body-cavity of the coelomates, is a dif- 

 ference in the walls of the cavities and not in the space between the 

 walls. The outer wall of the primary body cavity is merely ectoderm. 



It will be remembered that this primary body cavity serves both as 

 a digestive and circulatory system in the coelenterates. In the higher 

 animals, therefore, it may be said that the blood-vessels are really part 

 of the primary body cavity. 



In triploblastic animals the mesoderm does not form a completely 

 solid mass extending the entire length of the body. A slight cavity is 

 left in its center extending along the long axis of the organism. 



