1^2 TIi€ Iiisli Nahiralisi. J»b'» 



(a) the possession of a well-developed operculum, capable, 

 when fully withdrawn, of accurately fitting the orifice of the 

 shell, and, in a lesser degree, {b) the amplitude of the shell- 

 cavity as compared with the size of its inhabiting animal. Both 

 of these conditions are contributory to the one effect, the 

 retention in and around the animal of the moistuie indispen- 

 sible to the exercise of its vital functions. Thus the 

 comparative!}' small capacity of enduring exposure to air in a 

 drj" room which distinguishes P7irp2ira lapillus is probably due 

 to the imperfection of its operculum and the smallness of the 

 inner dimensions of its large and heavy shell. Of 170 full- 

 grown specimens of this species collected on granite rocks 

 above high-water mark in Januar}' last, 2, or \\ per cent., were 

 found to be altogether destitute of operculum, uhile the same 

 percentage had very imperfect opercula, covering only half 

 the animal's foot when fully withdrawn. Of 147 specimens 

 collected at the same time at low^ water, while not one was 

 found to be wholly destitute of operculum, 4 individuals, or 

 2| per cent., had this appendage very imperfectly developed. 

 Speaking generally, the operculum in this species is more or 

 less imperfect, so that it does not completely close up the 

 shell orifice when the foot is withdrawn. It ma}^ be that this 

 incipient degradation of the operculum in Purpura is a first 

 step on the long road towards total extinction of that ap- 

 pendage, as has been suggested in the case of the allied 

 genus Concholepas. 



Com.ing now to the first question, whether the artificial tests 

 emplo3'ed in the experiments here recorded are more or less 

 severe than the natural tests to which the littoral mollusca 

 may be submitted, it is clear that the question has no reference 

 to one of the tests, immersion in water of treble salinity, since 

 that condition does not occur in nature. As for the other 

 tests, exposure to dry air and immersion in perfectly fresh 

 water, the question is by no means easy to answ^er. All of the 

 littoral Gastropods here dealt with attach themselves b}^ their 

 partially extended foot to the rocks or seaweeds the}' inhabit. 

 While so attached they are obviously deprived of the protec- 

 tion of the operculum, and a considerable portion of the 

 animal's body is necessarily exposed to the air. But the more 

 rapid dissipation of the moisture in and around the animal's 



