138 ELEMENTS OF BIOLOGY 



a longitudinal partition, the mediastinum, into right and left pleural 

 cavities containing the right and left lungs. 



The Body Plan. Bilaterally symmetrical forms always tend 

 to develop so that the greatest dimension is in the median plane of 

 symmetry. The primary layers are thereby elongated. In triplo- 

 blastic forms the architecture is that of a tube-within-a-tube, the 

 body wall forming the outer tube, the digestive canal the inner 

 tube, and the ccelom the cavity between the two tubes. The inner 

 tube in the most simple triploblastic animals, for example, the 

 Platyhelminthes, is in reality not a tube but a more or less sac-like 

 structure having but a single opening, the mouth. In this they re- 

 semble the diploblastic animals. In all others the inner tube or 

 digestive canal has both a mouth and an anal opening. A simple 

 arrangement is found in some of the members of the phylum Nem- 

 athelminthes, the round forms (Fig. 41). Here the body wall and 

 the digestive canal are the same length, the cavity between being 

 continuous and filled with fluid and certain reproductive organs. 

 The body cavity of the Nemathelminthes differs in origin from that 

 of ccelomate animals, in that the mesoderm is wholly applied to the 

 under surface of the ectoderm and does not form the covering of 

 the digestive canal. 



Segmental Animals. In the members of the phylum Annelida 

 the ccelom is divided into a series of compartments by cross parti- 

 tions known as sept^. These compartments or metameres are occu- 

 pied by the coelomic fluid and in some regions by the expansions of 

 the digestive canal and by the organs of reproduction. This en- 

 croachment of organs into the ccelomic space is still more marked in 

 the members of the phylum Arthropoda. In the phylum Chordata, 

 particularly in the vertebrates, the ccelom is not divided by regularly 

 spaced cross partitions, the septcT being in evidence only in the body 

 wall. The great development of the digestive glands, such as the 

 liver, the great length of the digestive canal, and the relatively 



