240 THE SEA SHORE 



It appears that limpets are not great travellers, the appearance 

 of the rock from which they have been removed being such as to 

 point to a very long period of rest. Those on hard rocks are 

 generally situated on a smooth surface just the size of the shell and 

 generally worn slightly below the surrounding level by the constant 

 friction of the shell ; while others that have settled on very rugged 

 spots have their cones adapted to the irregular surface. It has 

 been suggested that the animals make occasional short excursions 

 from their chosen spot, but return again to it ; and whether or not 

 this is the case, it is evident that they frequently keep to one small 

 spot for a considerable length of time. 



Limpets on chalk and other soft rocks are sometimes in circular 

 pits so deep that even the apex of the shell is below the general 

 level around; and though it is possible that the abrasion is 

 produced entirely by the friction of the shell as the animal turns, 

 yet, in the case of chalk, the action may be partly due to the 

 carbonic acid gas given off by the animal as a product of respiration, 

 for it is a well-known chemical fact that this gas, in solution, has 

 the power of dissolving calcareous material. 



The other British Limpets include P. pellucida, which lives on 

 the fronds and stalks of the tangle, the form of the shell varying 

 according to that of the surface on which it rests ; also the Horse 

 Limpet (P. athletica), the bold radiating ribs of which are irregularly 

 notched ; and Acmcea testudinalis the Tortoiseshell Limpet, with 

 reddish-brown mottlings on the exterior, and a dark-brown patch 

 at the apex within. The last-named species lives principally on 

 sea weeds, and has a single pectinated gill in the cavity between 

 foot and mantle, which is protruded on the right side when the 

 animal is extended. This latter feature is interesting since it shows 

 a tendency to that one-sided development already referred to as 

 characteristic of the typical gasteropod, resulting in the spiral form 

 of the adult. 



In the limpets the lingual ribbon is proportionately long, and 

 is easily removed for examination. In P. vulgata it may exceed 

 an inch in length, and the teeth are arranged in rows each of which 

 contains four central, with laterals on either side, while in Acmcea 

 there are only three laterals on each side of the central line. 



Other so-called limpets belong to separate families. Thus we 

 have the Cup-and- Saucer Limpet and the Bonnet Limpet in the 

 Calyptrceidce. Both these differ from Patella in that the apices 

 of their shells show a tendency to assume a spiral form, thus 



