Fig. 166.—Patellidæ
1. Patella vulgata. 2. P. pellucida. 3. P. athletica. 4. Acmæa testudinalis
The Common Limpet (Patella vulgata) is found on all our rocky coasts between the tide-marks, often at such a level that it is left exposed to the air for eight or nine hours at a time. The apex of the shell of this species is nearly central, and the exterior is sometimes nearly smooth, but more commonly relieved by radiating ribs.
Although the shell itself is not a particularly pretty object, it is often rendered very beautiful and interesting by the various animal and vegetable organisms that settle on it. Those shells that are left dry for hours together are commonly adorned with clusters of small acorn barnacles, while the limpets that have found a home in a rock pool and are perpetually covered with water, often resemble little moving gardens in which grow beautiful tufts of corallines or other weeds, as well as polyzoa and other animal forms.
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 Acmæa 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 Acmæa 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 Calyptræidæ. Both these differ from Patella in that the apices of their shells show a tendency to assume a spiral form, thus denoting a somewhat closer relationship to the more advanced univalves. They have distinct heads, with prolonged muzzles, and well-formed antennæ and eyes. The teeth of the lingual ribbon are single, with dentated laterals on either side.