194 LESLIE B. AREY AND W. J. CROZIER 



sponds instantly to a touch upon the anterior or posterior girdle 

 by ceasing locomotion and adhering firmly to the substratum. 



B. Ventral parts, d. The ventral surface of the girdle. The 

 serial arrangement of eight dorsal shell plates affords a con- 

 venient means of dividing the surface of the animal into defi- 

 nitely delimited areas for reference. We shall consider the ven- 

 tral surface of the girdle in terms of four 'quarters,' — an 'ante- 

 rior quarter/ delimited by the posterior margin of the second 

 shell plate, two 'middle quarters,' and a 'posterior quarter' 

 correspondingly marked off by the transverse borders of each 

 succeeding two shell plates. The end quarters of the ventral 

 mantle are more reactive to tactile excitation than are the 

 middle quarters. 



a. The end quarters. To the single stimulation of an end 

 quarter, the response is a curling of the animal in that region, 

 as if it were beginning to roll up ; the bending process elevates the 

 stimulated end about 2 to 5 mm., after which the animal straight- 

 ens out again. When stimulated on the posterior quarter, the 

 foot may be pushed caudad at the time the bending response 

 occurs. When stimulated on the anterior quarter, the head may 

 retract somewhat, and the buccal region may be shghtly in- 

 verted, the head and 'palp' tending to close over the mouth. 



Light touches, when several times repeated, elicit a much 

 stronger response. The stimulated end reacts first, by bending, 

 and then the opposite end bends also, though to a less extent. 

 This is true whether the anterior or the posterior end is the one 

 stimulated. The muscles of the midportion of the animal are not 

 specially contracted, however, and the closure of the shell is 

 incomplete. 



jS. The middle half. The reactions listed under a are pro- 

 duced most clearly when the most anterior or the most posterior 

 region of the ventral girdle surface is stimulated. The reponses 

 obtained from the 'middle quarters' of this surface are qualita- 

 tively identical over the whole anteroposterior extent of the 

 ctenidia. In other words, the convenient descriptive division of 

 the animal into ' quarters' does not afford a basis for the organic 

 classification of responses, inasmuch as the ctenidia extend an- 



