136 INTRODUCTION TO THE COELOMATA [CH. 



cells in the primary body-cavity. To such cells the name meso- 

 derm has also unfortunately been applied and great ambiguity has 

 resulted from this practice. The term mesenchyme has been pro- 

 posed for these cells and it is to be hoped that it will be more 

 generally adopted so that the name mesoderm may be confined to 

 the coelomic wall. The origin of mesenchyme is different in different 

 cases. In Actinozoa it appears to arise mainly from the ectoderm, 

 in Ctenophora from the endoderm. In Platyhelminthes it arises 

 principally from the endoderm. In the higher Coelomata it arises 

 partly from the ectoderm but principally from the outer wall of the 

 coelom. Everywhere it gives rise to connective tissue and to 

 the tissues developed from this (tendon, cartilage, bone, etc.), 

 whereas the coelomic wall or true mesoderm gives rise to the gene- 

 rative cells and their ducts, and the main parts of the muscular 

 system including the muscular coats of the principal blood-vessels. 



The endoderm, after the separation from it of the mesoderm, 

 forms the lining epithelium of the digestive tube and of its ap- 

 pendages, which in the higher Vertebrata are the organs known as 

 lungs, liver, pancreas, and urinary bladder. The basis of the 

 skeleton of Vertebrata, the gelatinous rod called the notochord, 

 also arises from it. 



We have already seen that the ectoderm can be intucked both 

 at mouth and anus so that both in front and behind there is an 

 ectodermal section of the alimentary canal. The anterior of these 

 sections we have already learnt is called the stomodaenm and the 

 posterior the proctodaeum. In Crustacea these sections form by far 

 the greatest sections of the alimentary tube. 



The internal anatomy of the lower animals was first studied by 

 physicians and others who were primarily interested in human 

 anatomy. An unfortunate 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 signifies part of the 

 endodermic tube. The pharynx of an earthworm is the stomot 

 daeum, in a fish it includes both stomodaeum and the first part of 

 the endodermic tube. The term liver has also been much abused. 



The names taken from the anatomy of the higher animals 

 which are customarily used in the description of the alimentary 

 canal are as follows: mouth- or buccal-cavity, pharynx, 



