MONSTROSITIES OF LITTORINA RUDIS. 197 



due to this become the fittest unless by this deformity they 

 become in contradiction to their surroundings and should persist 

 rather than the normal form. Most colonies of monstrosities have, 

 as far as I know, been found in freshwater rather than in salt, and 

 the operating cause should be sought in some cause common to 

 both salt and freshwater. 



The only cause common to both is, I think, weed. I refer to 

 the effect produced by weed attaching itself to the shell in tufts or 

 covering it with a tangled mass. A microscopic growth was, it 

 should be remembered, noticed on some of the Planorbis found by 

 Dr. Baudon. There is undoubtedly a large quantity of it present 

 in the Fleet/ many of the living specimens on the shore having 

 tufts of it attached. Mr. J. T. Marshall, who has seen these 

 Littorinas, considers that the monstrosities are due to this cause. 

 He says : "I know of only one similar instance where Rissoa Parva, 

 otherwise fine large examples, have the last whorl constricted and 

 partially disconnected from the axis of the shell. It likes among 

 spore-like seaweed, and the young filaments of the latter attach 

 themselves to the shell and in some way incommode its inhabitant." 

 He also suggests as an explanation of the fact that one species is 

 affected and not the rest, that " while the sluggishness of one 

 species might tolerate its presence " i.e., that of weed " the 

 activity and livelier habits of another would keep them more or 

 less free from its bad effects." 



Why the Littorina does not being graminivorous eat off the 

 weed which inconveniences it I cannot say, unless this weed is in 

 some way distasteful to it. Possibly the animal when one of these 

 tufts presents itself in its way while it is forming shell either turns 

 off and alters the direction of the whorl, so as to avoid this tuft, or else 

 throws out a stratum of shell over it. If the former, a specimen 

 would be produced in which the relative height of the spire or the 

 direction of the whorl would vary ; if the latter, then one with a 

 tendency to become scalariform. If the cause continued a mon- 

 strosity would result, while if no more weed hindered the animal 

 might, as is the case with some specimens, return to the normal 



