IX] INTRODUCTION TO THE COELOMATA 137 



oesophagus, stomach or crop, gizzard, intestine, 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 literature that such a course would render 

 unintelligible most anatomical descriptions of species that we 

 possess. 



Besides forming the outer layer of the skin or epidermis of 

 the animal and the stomodaeum and proctodaeum, the ectoderm 

 gives rise to the brain and nervous system and to the essential cells 

 of the sensory organs. 



