526 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 62 



Schneirla, 1935, pp. 14-23) that in response to the stimulus of touch 

 on the amoeba a chemical reaction is produced that sets up currents 

 in the body protoplasm resulting in the extension of a pseudopod and 

 movement of the body. According to the strength of the stinmlus, 

 movement may be toward or away from the point of stimulation. 

 Forward movement is explained by Allen (1962) as due to contrac- 

 tion of the pseudopods. If this is animal life in its simplest mani- 

 festation, it is little more than chemical and physical reactions induced 

 in a plastic substance by external stimuli. Other protozoons have 

 become more complex in structure and have developed specific motor 

 organs in the form of vibratile cilia or flagella. 



The protozoons all move by some visible motor mechanism, but some 

 cells in the tissue of plants or animals have been described as simply 

 moving in a purposeful direction without giving any evidence of how 

 they do it. Several examples of this are given by Lewis (191:0) . One 

 is that of spicule-carrying cells in sponges, described by Schroder 

 (1936). The primary spicule is formed as an axial filament in the 

 cytoplasm of a cell, which then dies. Another silica-containing cell 

 now comes up and discharges its silica on the young spicule, which is 

 thus increased m thickness. Other cells then attach themselves to the 

 spicule and transport it to its proper place in the sponge body. It 

 would be useless to ask what motivates or operates these transporting 

 cells. 



A still more remarkable example of activity in tissue cells has been 

 ascribed to the flat worm Microstomum,, described by Lewis from 

 the studies of Kepner, Gregory, and Porter (1938). The worm eats 

 the green freshwater hydra and ingests its nematocysts (epidermal 

 cells with projectile threads). The hydra possesses four kinds of 

 nematocysts, in two of which the threads are stinging, a third kind 

 adhesive, and the fourth grasping. All four kinds of nematocysts are 

 taken into the stomach of the worm and penetrate through the 

 stomach wall mto the parenchyma. Here the nonstinging nemato- 

 cysts are digested and absorbed as being of no use to the worm. The 

 stinging nematocysts, however, are transported by certain parenchyma 

 cells to the epidermis, where they are oriented with the threads 

 directed outward. At the point of contact of each nematocyst with an 

 epidermal cell, the latter becomes pitted internally and externally to 

 allow the discharge of the nematocysts thread. Thus Microstomum 

 arms itself with the stinging nematocysts of the hydra as weapons for 

 its own defense. After 100 hours all ingested nematocysts are ready 

 for action, but when the epidermis if fully loaded, the superfluous 

 nematocysts are retained in the stomach and thrown out of the mouth. 



"Thus it becomes evident," the authors point out "that endoderm, 

 parenchyma, and epidermis of microstomum cooperate in the manipu- 

 lation of hydra's nematocysts." "In all this conduct," according to 



