154 MANUAL OF THE MOLLUSCA. 



Forbes has very happily suggested that the cyclo-branchiate gill of patella is, 

 in reality, a single, long branchial plume, originating on the left side of the 

 neck, coiled backwards round the foot, and attached throughout its length. 

 This view is confirmed by the circumstance that the gill of the sea-Aveed 

 limpets {nacelJce) does not form a complete circle, but ends without passing 

 in front of the animal's head. 



Patella, L. Eock limpet. 



Etpn., patella, a dish. Syn., helcion, JNIontf. 



Ex., P. longicostata, PL XI., fig. 22. 



Shell oval, with a sub-central apex ; sm-face smooth, or ornamented with 

 radiating strise or ribs ; margin even or spiny ; interior smooth. 



Animal with a continuous series of branchial lamellae ; mantle -margin 

 fringed ; eyes sessile, externally, on the swollen bases of the tentacles ; mouth 

 notched below. Lingual teeth 6, of which 4 are central, and 2 lateral ; 

 uucini 3. 



The tongue of the common British limpet (P. vulgata) is rather longer 

 than its shell ; it has 160 rows of teeth, ANdth 12 teeth in each row, or 1,920 

 in all (Forbes.) The limpets live on rocky coasts, between tide-marks, and 

 are consequently left dry twice every day ; they adhere very firmly, by at- 

 mospheric pressure (ISlbs per square inch), and the difficulty of detaching 

 them is increased by the fonn of the sheU. On soft calcarious rocks, like the 

 chalk of the coast of Thanet, they live in pits half an inch deep, probably 

 formed by the carbonic acid disengaged in respiration ; on hard limestones 

 only the aged specimens are found to have worn the rock beneath, and the 

 margia of their shell is often accommodated to the inequalities of the surround- 

 ing surface. These circumstances would seem to imply that the limpets are 

 sedentary, and live on the sea- weed within reach of theu- tongues, or else that 

 they return to the same spot to roost. On the coast of Northumberland we 

 have seen them sheltering themselves in the crevices of rocks, w^hose broad 

 surfaces, overgrown with nuUipores, w^ere covered with irregular tracks, 

 apparently rasped by the limpets in their nocturnal excursions* 



The limpet is much used by fishermen for bait ; on the coast of Berwick- 

 shire nearly 12,000,000 have been collected yearly, until their numbers are so 

 decreased that coUectiug them has become tedious (Dr. Johnston). In the 

 north of Ireland they are used for human food, especially in seasons of 

 scarcity; many tons weight are collected annually near the tow^n of Larue 

 alone (Pattison). 



On the w^estern coast of S. America there is a limpet which attains the 

 diameter of a foot, and is used by the natives as a basin (Cuming). 



* If limpets are placed in stale water, or little pools exposed to the hot sun. they 

 creep out more quickly than one would expect ; the tracks they leave are very peculiar, 

 and not likely to be mistaken when once seen. 



