AND OF PEODUCTS OF METABOLISM. 299 



say, the products of excretion of an Echinoid act more 

 adversely both on the death rate and on the growth of 

 embryos if these belong to its own species, than if they 

 belong to another species. At least this is the case 

 with Strongylocentrotus, Sphcer echinus, and Echinus. 

 With two other (physiologically) less closely related 

 species, viz., Arbacia pustulosa and Dorocidaris papil- 

 lata, it was even found that the products of excretion, 

 so far from acting adversely on growth, actually fav- 

 oured it. Thus Strongylocentrotus larvae grown in 

 water fouled by these two species were increased in 

 size by respectively 4.3 and 1.7 per cent., whilst respect- 

 ively 81 and 50 per cent, of the ova employed reached 

 the pluteus stage. 



It will probably be thought that this last result is 

 erroneous; but other observations showed that it was 

 not so. Thus Strongylocentrotus larvae were grown in 

 water fouled by various other animals, and it was found 

 that in this case also there was generally a distinct in- 

 crease in size. We see in the accompanying table that 



of the ten observations made, a positive effect (averag- 

 ing 4.1 per cent.) was produced in seven instances, 



