148 THURLOW C. NELSON. 



That such is not the case must be evident to anyone who has 

 observed extensive oyster sets on the bottoms of boats, or upon 

 the bottoms of floats kept at the very surface of the water 

 (Nelson, '17,). The full-grown oyster larva, so far from being 

 helpless, is capable of more powerful swimming than at any time 

 during its pelagic life. During the course of the development 

 and growth of the larva the increase in size of the velum is more 

 than commensurate with the increase in the weight of the shell. 

 Plankton samples taken during the calm of early morning show 

 large numbers of full-grown oyster larvae swimming at the sur- 

 face. At times the larva may project the foot into the surface 

 film and, withdrawing the velum, may hang suspended by the 

 foot and slowly rock the shell from side to side, much as in the 

 familiar habit of pond snails hanging from the surface film. 



Full-grown oyster larvae are found mostly at the bottom be- 

 cause they develop during the last two days of their pelagic life a 

 strong positive stereotropism, and since fixed objects are most 

 abundant on the bottom the larvae mainly congregate there. 

 The intensity of attachment which occurs at the very surface of 

 the water when suitable cultch is available, precludes the idea 

 that increased weight of the shell or even positive geotropism are 

 factors exclusively governing the time of fixation. 



The use of the foot by oyster larvae in crawling over surfaces is 

 similar to that reported by Crozier, '21, who found that the adult 

 Lima oriented itself away from the light by attaching the tip of 

 the foot to the substratum after bending the free portion away 

 from the source of illumination. The creeping of oyster larvae 

 is also strikingly similar to that observed in the fresh-water 

 Sphacridae, and especially in the young of Corneocyclas when first 

 liberated from the brood pouches of the parent (Nelson, '2I-B). 







In sessile forms, such as the oyster, which remain fixed on the 

 spot where the larvae set, keen competition follows when large 

 numbers settle at one time. During heavy sets 1,000 larvae may 

 attach to one oyster shell 8 cm. long, upon an area which will 

 permit not more than 6 oysters to grow to brtrding age. It is 

 significant that at such times of abundant settlement, when the 

 water over the oyster beds may show as high as 250 full-grown 

 larva; per liter, the frequency of attachment per unit area ol" 

 available cultch does not exceed a certain maximum. 



