540 Phylum Mollusca 



or heavy felt. Filtration of water supply is always advisable to eliminate 

 the larger competing or predatory forms. Oyster larvae have been 

 successfully reared in centrifuged seawater by Wells (1926) though 

 their development in this medium required about twice the normal time. 



The chief difficulty in rearing the microscopic, free-swimming larvae 

 of lamellibranchs is to prevent their escape in renewing the water supply. 

 Countless millions of embryos of the previously mentioned species, and 

 early, straight-hinge veligers (1 to 4 days old) may be obtained by 

 artificially fertilizing the ripe eggs in jars of slowly running seawater 

 from which the more healthy and vigorous swimmers are carried over 

 and concentrated in large funnels containing coarse filter paper. Col- 

 lections are taken from the filters for a period of 4 to 6 hours and trans- 

 ferred to aerated aquaria with an ample volume of water. If the 

 embryos are not overcrowded and obtain sufficient oxygen they will 

 develop into larvae with complete bivalve shells in 20 to 30 hours. After 

 reaching this stage they should have a complete change of water at least 

 every 24 hours and may be concentrated by the use of paper filters, high 

 speed centrifuge, or No. 25 bolting silk, provided the flow of water 

 through the latter is very gradual. The more vigorous individuals 

 may be selected by the same procedure as that used previously following 

 fertilization. In renewal of water, particularly in early operations, it 

 is always advisable to discard the small amount remaining in the 

 bottom of the aquaria in order to eliminate waste products, sediment and 

 any weak or dead larvae. As the larvae increase in size a slow but 

 continuous exchange of water may be maintained by filtering through 

 No. 25 to 20 bolting silk placed over the mouth of a large glass funnel. 

 The most critical period in development is after the larvae have com- 

 pletely utilized the yolk supply from the egg and begin to take in micro- 

 scopic food from the water. If suitable conditions are maintained as 

 to oxygen, water exchange, etc., during this interval a fair percentage 

 of the larvae will survive and may be reared through the succeeding 

 stages without much difficulty. 



It is a comparatively simple matter to maintain cultures of larvae 

 that are half to full grown (approximate diameter 0.2 to 0.5 mm.) 

 as they may easily be concentrated or retained by No. 18 to 14 bolting 

 silk or by monel metal wire cloth or sieves of 200 to 80 mesh. Any toxic 

 effects of the metal cloth may be prevented by a thin coating of celluloid 

 though the author has successfully used the uncoated material after it 

 has been "seasoned" in seawater for a few days. The essential require- 

 ments for the advanced stages are a continuous or twice daily exchange 

 of filtered water and gradual circulation by air or a slow stirring device. 

 The procedure outlined previously is essentially the same as that used 

 by Prytherch (1924) and Wells (1926) in the culture of oyster larvae 



