316 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



invertebrates play an important part in the food supply 

 of other animals, particularly of young fish ; but still they 

 are not so important in this respect as are the Copepoda. 

 In the first place, the number of invertebrate larva?, Cope- 

 pod larvae being excepted, is never so large as the average 

 number of Copepoda in a similar volume of water ; and 

 secondly, these larva? are only present in enormous 

 quantities at certain periods of the year, that is, when the 

 spawning period of the adult occurs, and so belong to the 

 " periodic plankton " in contradistinction to the Copepoda, 

 which are always present, and therefore belong to the 

 perpetual plankton. 



The microscopic Infusoria, which are included under 

 the title Tintinnse, also belong to the periodic plankton. 

 Their number is so enormous that they play an important 

 role in the plankton. The principal Baltic form, 

 Tintinnus subulatus, occurs to the number of 1,228,000 in 

 10 cubic metres of water. During the months when they 

 are at a maximum their number equals that of the Cope- 

 poda, but as they are very much smaller, their total mass 

 is much less. 



The Peridinese are also present in enormous quantities. 

 The commonest form is Ceratium tripos, which is the usual 

 cause of phosphorescence in the Baltic. The numbers 

 for 10 cubic metres of water are, maximum 130 million, 

 minimum 44,000, average 14 million. 130 million per ten 

 cubic metres gives 13 for a cubic centimetre ; the average 

 gives 1'4 per ccm. The Peridinese are of importance, 

 since they presumably form the chief source of the food 

 supply of the Copepoda. The food supply of the latter is 

 not, however, definitely known. Hensen and other 

 investigators found no determinable substance in their 

 alimentary canal, but only a mass of green chlorophyll- 

 containing material. 



