no ANIMAL AGGREGATIONS 



to which snails grow depends in some way on the actual volume to 

 which they are exposed and on the surface area of such water. His 

 explanation was that in the small tubes the snails needed to move 

 about less to obtain their food and that, with this decrease in exercis- 

 ing, there came a decreased rate of growth. According to De Va- 

 rigny, dwarfing from crowding is not so much due to the actual 

 numbers in the vessel as to the "psychological" influence of num- 

 bers, which inhibit exercise, just as a man is less likely to walk a 

 considerable distance on a crowded street than on a deserted one. 

 He also believed dwarfing to be affected by the accumulation of 

 faeces. 



Willem (1896) bubbled air through his snail cultures and found 

 growth of the contained snails greatly increased. He concluded that 

 aeration is important because, even in lung-breathing pond snails, 

 he believed cutaneous respiration to be more important than lung- 

 breathing and alone sufficient for the animal. 



Davenport (1899) reviewed much of the evidence on the relation 

 between crowding and rate of growth, and concluded with Hogg that 

 in respect to the size attained, as in other qualities, the snail has the 

 power of adapting itself to the necessities of its existence. 



Vernon (1895, 1899, 1903), working with echinoderm larvae, con- 

 cluded that dwarfing is due to a concentration of the excretory prod- 

 ucts in the media. He found that if eggs of echinoderms were allowed 

 to develop in water which had previously contained other eggs for a 

 considerable period of time, the larvae of the second batch were dim- 

 inished in size as compared with the control. The growth of the 

 larvae appeared to be reduced by their own excretory products, or 

 especially by those of adult echinoderms, the more so if these belonged 

 to the same species. On the other hand, he found that the excretory 

 products of two species not closely related were favorable to 

 growth. 



Warren (1900), working with the common entomostracan, Daph- 

 nia, found that continued breeding in small aquaria with the medium 

 unchanged caused dwarfing. This result he attributed to the action 

 of the excretory products, which he found to be somewhat specific, 

 since ostracods and copepods flourished in cultures of Daphnia in 



