206 THE STUDY OF ANIMAL LIFE CHAP. 



internal affairs. Or we may classify them according to 

 their development from the outer, middle, or inner layer 

 of the embryo. Thus the brain and the essential parts 

 of the sense-organs are due to the outer ectoderm or 

 epiblast ; muscles and skeleton arise from the middle 

 mesoderm or mesoblast ; the gut and its outgrowths such 

 as lungs and liver originate from the inner endoderm 

 or hypoblast. Or we may arrange the various structures 

 more or less arbitrarily for convenience of description 

 as follows : the skin and its outgrowths, appendages, 

 skeleton, muscular system, nervous system, sense-organs, 

 the food-canal and its outgrowths, the body-cavity, the 

 heart and blood-vessels, the respiratory organs, the 

 excretory system, the reproductive organs. 



Of the order in which organs appear or have appeared 

 we can say little. The simplest sponges and polypes are 

 little more than two-layered cups of cells, the cavity of 

 the cup being the primitive food-canal. A parallel stage 

 occurs in the early life-history of most animals, when the 

 embryo has the form of a two-layered sac of cells, i.e. in 

 technical language, a gastrula. Both in the racial and 

 individual life-history the formation of this primitive 

 food-canal occurs very early. But it is not certain that 

 it the primitive stomach was not at a still earlier stage 

 an internal brood-cavity ! 



6. Correlation of Organs. One of the most funda- 

 mental facts about organs is their correlation. They are 

 members one of another, they stand or fall together, 

 they are physiologically knit, they have been evolved in 

 company. Thus heart and lungs, muscles and nerves, 

 are closely correlated. Sometimes it is obvious Avhy two 

 or three structures should be thus connected, for it is 

 of the very essence of an organism that its parts are 

 members one of another. In other cases the reason of 

 the connection is obscure. Influences pass from one 

 organ to another by the nervous system. Thus a diges- 

 tive disturbance may affect the eye or the heart. Or the 

 influence may be conveyed chemically by the blood, and 

 of great importance in this connection are the ' ' internal 

 secretions ' or hormones which are produced by duct- 



