124 



occuiTence is the develupiuent of a fiiiiow pioeeeding 

 from a small depression on the siiiface of the embryo, 

 which is the early shell g-land. 



A velum is developed as a circular ridg-e covered with 

 long' cilia, and encircling the embryo between the 

 blastopore and the shell gland. The stages following 

 have no't been observed in F. oijcrculafu, but probably 

 there are one or two long apical cilia developed in the 

 centre of the velum, and the embryo elongates in the 

 direction of the axis of these cilia, though in the stages 

 which Fullarton observed with the shell valves developed, 

 there were neither apical cilia nor flagellum present. A 

 trochospere larva is thus formed, with a velum but no 

 shell. Later the mantle folds are developed and two 

 shell valves, and the veliger stage is entered upon, 

 locomotion still being carried on by the velum, which is 

 extended beyond the margins of the valves and is retracted 

 by two velar muscles. A single adductor, the anterior, is 

 developed, and the alimentary canal formed ; the mouth- 

 opening being just posterior to the vehim, and the anus 

 close to the mouth. 



ECONOMIC IMPORTANCE. 



Pecten opercularis occurs in large beds which are 

 found in many localities round the British Isles. In 

 most O'f these places little use seems to be made of the 

 scallops, though they might be of considerable value both 

 as food and bait. The species occurs in such abundance 

 in the Firth of Forth that an i,mportant industry has 

 arisen, which once employed a considerable number of 

 hands, tbough in recent years the number has diminished. 

 In Scotland, P. ojjercularis is known as the " clam," 

 though it must be remembered that this name has been 



