THE SHELLFISH INDUSTRY 309 



with the even production of the shell. Mussels and other 

 bivalves show similar " disturbance " rings but not always 

 winter rings. Oysters increase their shells by sudden 

 bursts of activity, depositing a broad but very thin layer 

 of new shell round the edge at a single operation. This 

 thin layer is called a " shoot " and quickly thickens and 

 hardens. The number of " shoots " per annum varies, 

 but in this country appears to be between two and four, 

 each of which in an oyster of two to three inches in depth 

 may mean an increase of about one-third of an inch all 

 round the edge. The variable number of these " shoots " 

 makes it quite impossible to judge the age of an oyster 

 unless the local conditions are very well known. 



Hitherto all the molluscs we have considered have been 

 bivalves, but there are also a number of univalve molluscs, 

 marine members of the snail family, which are used as 

 food. Of these perhaps the periwinkles (Littorina) are the 

 most important. They are common on rocks and stones 

 between tide-marks from which they are easily gathered 

 at low tide. They are usually boiled before they are eaten. 

 The common whelk (Buccinum undatum) is much larger 

 than the periwinkle though in other ways very similar. 

 It is not very common on the shore but is taken by dredging 

 and to some extent in pots. It has a strong white or 

 yellowish shell, good sized specimens being some four 

 inches long by two and a quarter inches broad. The 

 animal within is correspondingly large, but the flesh is 

 close and tough. Nevertheless, after boiling or pickling, 

 they are eaten in tremendous numbers. The common 

 limpet (Patella vulgata) is also collected and sold as food 

 in many parts, but is extremely tough. 



Molluscan shellfish provide a valuable source of food. 

 Owing to the fact that they store great quantities of 

 glycogen or " animal starch " as well as fat and protein, 



