276 THE BIOLOGY OF THE SEA-SHORE 



intrinsic movement caused by a pulsating vesicle situated 

 at the opening of the oviduct, which partly surrounds the 

 ribbon and throbs rhythmically as it slowly pays the latter 

 out (according to Trinchese and Hecht, the number of 

 pulsations is from 24 to 56 per minute). In Doris tuber culata 

 the rate of spawning is a little more than ^ inch per hour. 

 (A specimen at Plymouth laid 15 inches in 24 hours.) The 

 ribbon passes backwards on the right side of the animal 

 between the mantle and the foot and thus receives a curve 

 along the whole of its length, the concave side facing out- 

 wards. The edge which lies nearest to the foot is attached 

 to the rock, and the curl of the spiral is counter-clockwise 

 (occasionally clockwise)." 



While the spawn of British species is nearly always white 

 (at most tinged with rose or yellow), that of tropical Dorids 

 is deposited in very beautiful red or violet spirals which 

 look like flowers. 



Eliot puts the duration of life of these molluscs at about 

 a year. The young are hatched as veligers in spring or 

 summer (England) and can reach their full size, as animals 

 from 2 to 3 inches long, in less than twelve months. In 

 the experiment made by Orton already referred to, a raft 

 moored at sea for six weeks was found to be covered with a 

 growth of Obelia geniculata on which adult Nudibranchs, 

 chiefly Galvina picta, were feeding. Masses of spawn 

 belonging to this species were found and from these free- 

 swimming veligers were given off. Orton concludes that 

 these Nudibranchs had peopled the raft as veligers, rushed 

 through their development at the expense of the hydroids, 

 and were giving off veligers again to populate hydroids 

 elsewhere within a period of not less than six weeks and two 

 days ! 



The spawning habits of several species of shore bivalves 

 have been extensively studied and are of considerable 

 commercial importance, but from our particular point of 

 view they show no features of outstanding interest beyond 

 the fact that they are all enormously fecund. The common 

 Oyster {Ostrcea edulis) produces from 600,000 to 1,800,000 



