SEA-FISHERIES LABORATORY. 181 
plaice anddab. There is said to be enormous destruction 
by the larger sea-birds. According to the fishermen who 
gave evidence to the Commissioners of 1879, the decrease 
in some cockle beds may be: directly attributed to the 
increase in sea-gulls due to the operation of the Sea-Birds 
Preservation Act. Whole cockle beds may be destroyed Be 
a hard frost or by an encroachment of sand. 
At the beginning of the year the reproductive glands 
are nearly ripe, and spawning commences at the end of 
February or the beginning of March. The spawning 
period is prolonged, and ends about June or July. The 
reproductive glands then pass into the ‘‘ spent’”’ condition 
and after a short time begin to ripen again. Eggs and 
milt are shed freely into the water, where fertilisation and 
development go on. The minute larva swims freely in the 
water for some time, then settles down in the sand as a 
small shelled cockle. 
After a detailed account of the anatomy of the cockle as 
a typical Lamellibranchiate Mollusc, an Appendix follows, 
containing an account of the animal from the economic 
aspect, with special reference to the cockle fisheries in the 
Lancashire Sea-Fisheries district. 
THE SHELL. 
As in the case of most Lamellibranchs, the shell is 
equivalve, that is, the two valves are precisely alike in 
shape; it is inequilateral, the straight central rib or groove 
on each valve dividing it into unequal anterior and posterior 
parts; as regards the form and relative proportion of 
the two parts so delimited, and the ratio of length to 
lateral breadth or depth, great variability is found. The 
number of ribs varies from 20 to 24. The external surface 
is marked by a series of fine concentric grooves and ridges 
indicating the growth of the shell margin. Some of these 
