THE SENSORY RESPONSES OF CHITON 191 



which there is a hole, provided the hole be situated beneath the 

 gill channel. 



During its normal existence, however, the foot is of course the 

 organ whereby the chiton maintains its position. The whole 

 girdle is often, especially when under water, completely removed 

 from contact with the substratum; the support of the animal 

 depends, in fact, almost entirely upon the foot. From the ease 

 with which the chitons preserve their position in places where 

 wave action is considerable, and upon the under surface of rocks, 

 or (as has been noted through continuous observation) upon the 

 relatively smooth vertical wall of a concrete wharf for periods 

 of more than five months, it will be seen that the working power 

 of the foot is, after all, adequate for the creature's needs. Chiton 

 gets the most possible out of this suction power of its foot by 

 keeping its whole area closely pressed against the substratum. 

 Since it commonly inhabits smooth rock surfaces, the foot usu- 

 ally exhibits no great unevenness when the animals are freshly 

 examined; but they are occasionally obtained creeping over bits 

 of stone or groups of small Modiolus or barnacles, and if these 

 individuals are inspected it is to be noted that the whole surface 

 of the foot has been thrown into blebs and deep depressions cor- 

 responding closely to the unevenness of the substratum. Some 

 of the blebs produced under these circumstances clearly demon- 

 strate the basic mechanical principle upon which the foot works, 

 for they appear as thin-walled vesicles filled with fluid (in the 

 females, orange in color like the coelomic juice). 



Similarly, if a Chiton be caused to creep over a small hole (4 

 to 5 mm. diameter) in a glass plate, the substance of the foot 

 will be perceptibly pressed into the hole. Apparently the foot of 

 chiton can exert suction only in a very local fashion, for if a 

 portion of glass tubing of 8 mm. internal diameter, 10 mm. 

 external diameter, corked at one end so as to provide a cylindrical 

 chamber 4 mm. high, be used to test the sucking power of the foot, 

 it is found that in most cases the chitons cannot become attached 

 to the circular rim of the tube with sufficient force to bear their 

 own weight in air. In this experiment, it should be noted, the 

 total area available for direct contact with the foot is about 28 



