40 LIVERPOOL MARINE BIOLOGY COMMITTEE REPORT. 



two cases, i.e, the possibility of obtaining new supplies of 

 food, and of escape from enemies. 



One of the most characteristic features of fresh water 

 forms, as compared with their marine allies, is the large size 

 of the eggs of the former. Thus, a crayfish, though but a 

 third of the length of a lobster, has actually larger eggs. 

 The explanation of this is probably to be found, as Professor 

 Sollas has suggested, in the following considerations. 



The effect of increased size of the egg is that the young, 

 having a larger supply of food in the egg itself, hatch of 

 larger size and greater perfection of development than would 

 be possible were the egg smaller. Hence, in forms with 

 large eggs, the earlier larval stages will be passed in the egg 

 before hatching. This is of special advantage in the case 

 of fresh water forms, firstly, because larval forms of very 

 small size would be unable to withstand the currents in the 

 streams and rivers in which they dwell, and so would be 

 carried down slowly but surely to the sea; and secondly, 

 because these earlier larval forms representing stages prior to 

 the acquisition of a fresh water habitat, would not be suited 

 to it, and might very conceivably be unable to live in it. 



These considerations offer a ready explanation of such 

 points as the passing of the early stages of development in 

 the gills of the mother in the case of the fresh-water Anodon, 

 and the subsequent attachment of the glochidia larvae to fish, 

 until they have attained sufficient size and strength to with- 

 stand the currents of the streams they inhabit. 



Finally, the relation of the shallow water to the pelagic 

 or oceanic fauna remains to be considered, and on this point 

 it is very difficult to speak with certainty. 



On the one hand, we find that in the pelagic fauna a com- 

 paratively small number of groups are represented. Sponges 

 and Echinoderms are absent ; there are but few pelagic 

 worms, and of Arthropods, Molluscs and Vertebrates, only 



