88 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



size, somewhat insect-like appearance and legs, and their 

 active movements, some of them are often called " water- 

 fleas." 



The most important group of the Entomostraca is the 

 Copepoda. These little " water-fleas " are exceedingly 

 abundant in the sea, and are of great economic import- 

 ance. It has been calculated that under each square 

 metre of the surface of the Baltic there are one million 

 Copepods, and that these use annually 4,730 millions of 

 the small Ceratium (see before) as food. Herrings feed 

 largely on Copepoda, and for a square mile of surface 

 water it has been shown that the annual consumption of 

 Copepoda is nearly 1,000 billion. Now a billion of Cope- 

 poda yields not less than 1,500 kilograms of dry organic 

 substance, and consequently in the 16 square miles of a 

 certain Baltic fishery the German biologists consider there 

 exists Copepod food for over 530 millions of Herring of an 

 average weight of 60 grammes. 



Two common Copepods are shown in fig. XII., 

 Temora longicornis at 9, and P seudocalanus elongatus at 

 12. Each of these is about \ of an inch in length. Mr. 

 Isaac C. Thompson, Hon. Treasurer of the Committee, is a 

 well-known authority on the Copepoda, and has written 

 many reports and papers published in our volumes. 



When a fine muslin or silk net is drawn through the 

 water of Port Erin Bay for a few minutes, at almost any 

 time of the year, a number of small free-swimming plants 

 and animals are captured. Such organisms are known 

 collectively as " Plankton.'' Some of the more minute 

 and simple of these (Protozoa and Diatoms) were shown in 

 fig. I., while a number of the commoner and larger 

 kinds of kk Plankton " are represented in fig. XII. They 

 are evidently a mixture of young and old belonging to 

 different groups. 



