190 LESLIE B. AEEY AND W. J. CROZIER 



That the essential nervous mechanism of progression is locally 

 contained within the foot is shown by the fact that when com- 

 pletely excised the foot will exhibit spontaneous wave motions ; 

 usually such a foot (which will live for two to three days in sea- 

 water) does not actually creep for more than a centimeter. The 

 isolated foot reacts locally at its margin and on its ventral sur- 

 face to touch, in the latter case giving well-defined suction re- 

 sponses. The pedal waves formed by the isolated foot are normal 

 as to their speed of transmission ; moreover, they appear one at 

 a time, in succession, as in ordinary creeping; usually two or 

 three waves exhaust the foot for half an hour. 



The foot of placophorans, as of gastropods, serves also as a 

 holdfast (Parker, '11), either by means of slimy secretions or 

 through the action of the foot as a sucker. Parker ('14) pointed 

 out that in Chiton tuberculatus the foot sucks locally, so that 

 "if to the foot of an inverted chiton a rigid body with an area 5 

 mm, square is applied, the animal can attach itself to this area 

 wdth sufficient strength to allow its weight to be lifted." As we 

 shall point out subsequently in this paper, this 5x5 mm. area 

 is about the minimal surface to which the Chiton foot will react 

 by attachment and suction, so the full physical efficiency of its 

 suction cannot, perhaps, be measured in this way. A chiton of 

 8 cm. length weighs approximately 50 grams, so in Parker's 

 experiment just cited the foot was probably exerting a suction 

 pressure of not more than 2 grams per sq. mm. — considerably 

 less than the almost perfect suction efficiency of the tubercles 

 upon the column of Cribrina (Parker, '17 e). 



These observations indicate that, although the chiton foot is 

 employed as a hold-fast, the foot itself is not sufficient to account 

 for the full suction power of these animals. The tenacity with 

 which they adhere to a rock surface is sufficiently remarkable to 

 have gained for them the local name 'suck-rocks,' and in a pre- 

 ceding section we have shown how the girdle is of prime impor- 

 tance in this connection. An individual from which the girdle 

 has been completely removed may with relative ease be sepa- 

 rated from a stone over which it has been creeping. This is also 

 true if a chiton is caused to become attached to a glass plate in 



