18 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 (1) @ modified sensory 
epithelium or layer of cells, to which is added (2) a more or 
less complex accessory apparatus. The epithelium is in 
every case directly connected with part of the nervous 
system, when this is present. 
Excretory System.—Excretory organs are of several 
types. We can usually recognise (1) 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 gi//s 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 sowards 
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 zevvous and vascular systems. The former isa 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—(1) the BLOOD VASCULAR 
