28 GENERAL BIOLOGY 



free cells, " corpuscles," floating in the liquid plasma. 

 In vertebrates the majority of these cells are of 

 definite shape and are the carriers of the charac- 

 teristic red pigment of blood. Such corpuscles are 

 also found in a few worms and other lower organisms. 

 In addition to these there occurs in nearly all meta- 

 zoans in which there is any blood or body-fluid 

 another sort of free cell, leucocytes or amoebocytes, 

 distinguished from the former by the lack of red 

 pigment and especially by the absence of any defi- 

 nite shape or bodily outline. If we examine the blood 

 of an earthworm or a crayfish with a microscope, 

 we may study these cells with comparative ease. 

 If the plasma containing such cells be kept slightly 

 warm, these leucocytes will be found to change form 

 continually. Short processes flow out from the cell- 

 body in different directions, and the rest of the pro- 

 toplasm appears to flow or be pulled along after them. 

 In this way the cell is able to progress slowly over 

 the slide of the microscope .or over the walls of the 

 blood-vessels in which it normally occurs. In other 

 words the cell possesses the function of locomotion. 

 The lobelike processes (called pseudopodia or " false 

 feet ") are protruded at any part of the cell-body 

 or on several parts at the same time. This function 

 of locomotion is therefore unlocalized. The surface 

 of the cell appears to be somewhat sticky (viscous) 

 and retains a hold on solid objects. When the cell 

 is creeping, a pseudopodium sticks in this way to 

 something solid, the protoplasm then contracts, and 

 the rest of the cell is pulled along with a flowing 



