i8 > . EXCRETORY ORGANS. 



The structure of these sense-organs will be dealt with 

 in each type, but we may here note that they resemble 

 each other in consisting essentially of (i) a 77iodified sensory 

 epithelimn or layer of cells, to which is added (2) a 7?wre or 

 less complex accessory apparatus. The epithelium is in 

 every case directly connected with part of tfie nervous 

 system, when this is present. 



Excretory System. — Excretory organs are of several 

 types. We can usually recognise (i) an excretory surface 

 which by its secretory activity produces the waste products, 

 (2) a duct to the exterior often endowed with motor cells to 

 carry the waste products to the surface of the body, (3) a 

 reservoir for the accumulation of the waste products before 

 ejection. All these parts can be distinguished in the series 

 from the simple contractile vacuole to the flame-cell organs, 

 the nephridia and the kidneys. 



The Respiratory organs are late in development. In the 

 lower animals, the surfaces of the body serve to effect the 

 interchange of oxygen and carbonic acid, but respiratory 

 organs, in the form of gills, arise from the worms onwards. 

 These gills are formed on the *' plant" principle of maximum 

 (respiratory) surface and minimum bulk and are usually 

 formed from the outer surface of the body. They are 

 replaced in land-animals by air-breathing organs of quite 

 another type. Air is usually taken into the body towards 

 the respiratory surface and pulmonary organs do. not pro- 

 trude from the body. Air is so much more mobile than 

 water that the greatest economy is effected in this way. 



Correlative Systems. — These four primary systems 

 are in intimate contact and relation with each other in the 

 lower types in which the functions are co-extensive with the 

 protoplasm of the body, but in the higher types the systems, 

 developed in each case in the most suitable parts of the 

 body, become removed from each other and systems of 

 correlation are necessary. The two most important of these " 

 are the nervous and vascular systems. The former is a system 

 of correlation between the sense-organs and the motor system, 

 whereas the vascular system connects all the others. 



Vascular Systems. — The vascular system in the higher 

 animals is usually of two kinds — (i) the blood vascular 



