3o6 SoiitJicyu cyoss. 



siK'ciiil brood cluinibur.s in AcLiiiianis is limited to Arctic and 

 Anlavclic species, there certainly is reason to believe that sonic 

 common conditions of environment have brought about their 

 development. Among Ecliinoderms, species from Kerguelen Island 

 anil other points in the southern and northern oceans have been 

 shown to possess similar chambers for the protection of the 

 developing embryo. lUit in this group, not only are there in these 

 species special nurseries formed, but the embryos themselves 

 develop ilircdlji, without the intervention of a locomotive pseud- 

 eml>ryonic stage, and with no trace of pseudembryonic appendages 

 or provisional organs, so characteristic of echinoderm development as 

 we are acquainted with it in our ow^n seas. In Sir Wyville 

 Thomson's words (9 p. 245), "It is a significant fact that while in 

 warm and temperate seas ' plutei ' and ' l)ii)innaria^ ' are constantly 

 taken in the surface net, in the southern seas they are almost 

 entirely absent." There is therefore some justification for Kwiet- 

 niewski's suggestion (o p. 122) that the surface ice affects the 

 plankton and is especially fatal to free-SMimming larva-, and hence 

 it becomes necessary that special protection should be provided, so 

 that development may take place without free-swimming larval 

 stages. But viviparous Actinians, in which the ccelenteron acts 

 as a brood chamber, and where the young are only set free when 

 they are in a position to at once attach themselves by the muscular 

 foot disc, are fairly widely distributed. It is therefore difficult to 

 advance a reason why these i)articular Arctic and Antarctic species 

 have not followed the apparently more economical habits of their 

 fellows, but have evolved along special lines, and formed brood 

 chambers distinct and entirely separated from the ccelenteron. It 

 may be that the young in these special chambers are retained for 

 a much longer period than would be convenient in the calenteron, 

 and this, no doubt, is of considerable importance to the species when 

 possibly the struggle for existence is severe. 



In conclusion I wish to record my appreciation of the kindness 

 of the Committee of the Liverpool Museums in granting mo 

 permission to use the Museums' laboratory and apparatus in carrying 

 out the work, and to express my thanks to Dr. Forbes, Director of 

 Museums, for lus cordial co-operation in procuring me access to the 

 necessary literature — always a very great difficulty to workers in the 

 provinces. 



