LARVAL OR SWIMMING STAGES 29 



large groups of animals (Rotifera, Turbellaria, Nemertea, Annelida, 

 Gephyrea, Echinodermata, Bryozoa, Brachiopoda, Mollusca, &c.), but 

 the veliger is peculiar to Mollusca only. A veliger is an advance on a 

 trochophore, not only in the possession of a shell, but also in the conversion 

 of the simple prototroch into a velum, which is a more efficient swimming 

 organ, capable of being folded and withdrawn into the shell for pro- 

 tection. The oyster larva is a veliger from the time of origin of the shell 

 throughout the rest of the oyster's larval or free existence. During this 

 period it grows to more than five times its original length, i.e. more than 

 one hundred and twenty-five times its original cubic contents, and under- 

 goes a corresponding advance in organization, The new organs, being 

 soft cellular structures, are, as a rule, inconspicuous, at least until some 

 time after their origin, so that they are ill-adapted to serve as land-marks 

 in dividing off stages or periods for accuracy of description. For this 

 purpose the shell is the most serviceable, because of its permanence, its 

 regularity in size and shape, and its growth in correspondence with the 

 soft parts. 



The shell first becomes recognizable (Plate I, fig. 10) when the larva 

 is about 10 units (=10 x 6.9 /i.= .069 mm.) in length. As already in- 

 timated, the age of the individual is too variable to be of much use in com- 

 parisons, varying with a northern or southern climate, a sheltered or ex- 

 posed locality, depth of water, &c., i.e., with the temperature; but, since 

 an approximation is better than nothing, I will select a single example. 



Shediac, N.B., Aug. 2, 1909. Surface sea-water 22 . 5° C. (= 72 . 5°F.) ; 

 S. G. 1.018. 11 oysters examined furnished two small ripe females and 

 one male. Eggs and sperms mixed at 10:05 a.m. Kept in sea-water in 

 tumblers aboard the "Ostrea" and at 5:30 p.m. next day, at Bay du Vin, 

 contained larvae with minute shells. 



At first each shell-valve appears as a small glistening spot on the side 

 of the soft-bodied larva, near its dorsal surface. It grows larger, covering 

 more and more of the body (fig. 11), becomes connected with its mate of 

 the opposite side along the hinge-line, and together they extend down- 

 wards and increase in length until they can enclose all of the larva except 

 the velum (fig. 12). At this time the shell measures 10 units in length 

 and 8 in depth, with an almost straight hinge-line of 7 units (10x8: 7), 

 from the ends of which it curves equally in front and behind. It is narrow 

 from side to side, its two valves are of equal size and shape, concave in- 

 wards, convex outwards, thin, gray, and transparent. The larva is 

 very inactive, except when swimming, but twitching movements of the 

 valves and of the velum show the presence of adductor and retractor 

 muscle-fibres. There is an oesophagus and a short intestine, and the 

 liver is forming from the stomach (Plate V, fig. 26). 



Up to this time the larva has not increased much in size beyond the 

 original egg, the protoplasm of which has furnished food and energy for 



