156 THE NEGATIVE GEOTROPISM OF THE PERIWINKLE 



factor is "the relative proportions of the anterior and posterior 

 regions of the animal's body. All the conditions being the same 

 it is this factor which determines whether the head end will be 

 directed up or down." 



These facts were demonstrated in more critical terms for 

 Littorina littorea (L) the common periwinkle, by Kanda (1916-2), 

 who, noticing how a periwinkle sank in water, observed that 

 the centre ofgravity for living animals and also for shells is located 

 in the posterior region of the shell. He determined in quantita- 

 tive terms the direct correspondence between the proportion 

 of animals exhibiting negative geotropism and the active com- 

 ponent of gravity. 



It is true also that there are pronounced anatomical modi- 

 fications associated with negative geotropism. Pelseneer (1895) 

 has shown that in the periwinkle, morphological modification 

 and specialization in the respiratory organs has been associated 

 with this activity. Part of the wall of the pallial cavity for 

 example, has become vascular like that of the pulmonary 

 chamber of the true Pulmonata. Thus the climbing habit 

 would appear to be normal and characteristic of the species. 



Tattersall (1920) records an attempt made in Ireland to 

 turn the climbing habits of the common periwinkle, Littorina 

 littorea, to practical use by the erection of stakes along the appro- 

 priate part of the littoral zone, on which the periwinkles might 

 ascend and so be collected more easily for marketing. While 

 the observations are deficient in certain respects, they demon- 

 strate several points of interest. The number of periwinkles 

 climbing the stakes varied from 15 to 35 per cent of the total 

 number on the area investigated. Calm water appeared to be a 

 necessity if any periwinkles were to climb, slight agitation of the 

 water around a stake being detrimental to a successful ascent, 

 loosening the slight purchase a periwinkle is able to obtain on a 

 comparatively rough climbing surface. 



Tattersall summarizes his results as follows: "From mea- 

 surements made on the climbing winkles and those found on 

 the ground at the same time, there appears to be no evidence 



