THE PELAGIAL 237 



to the size of pteropods, form the food of the larger pelagic animals. 

 In order to secure their food, which is minute as compared with them- 

 selves, the plankton feeders require special apparatus which has been 

 convergently developed from different origins into similar structures 

 with like functions. 



Lohmann 19 divides the plankton feeders into three groups, those 

 which feed by means of tentacles, those which produce a current of 

 water, and those which hunt actively. The most primitive type is that 

 of the tentacle feeders, which feel about for their food while remain- 

 ing at rest. The radiolarians and Foraminifera do this with their 

 radiating pseudopods, which paralyze their minute food by a poison. 

 The long tentacles of siphonophores (up to 30 m.) serve the same 



Fig. 60.— The appendiculate Oikopleura albicans (black) in its house. To the 

 left of the animal is the permanently outspread net. The black arrows show 

 the direction of the currents of water produced by the undulation of the tail. 

 The light arrow beneath points in the direction in which the animal moves. 

 After Lohmann. 



purpose. The cephalopod Chiroteuthis has its sucking disks trans- 

 formed for this purpose into sticky threads, from which the food is 

 removed by the specially adapted lips. 20 



The most singular means of securing food is that of the animals 

 which strain out the living forms from a stream of water which they 

 produce themselves. Nets or screens are produced for this purpose. The 

 bristles of the mouth parts of Cladocera and copepods (Fig. 59), the 

 slime bands in the body cavities of salpas and pyrosomas, the remark- 

 able apparatus of the appendiculates, built up out of jelly-threads 

 (Fig. 60), the screen-like gill strainers of the plankton-eating fishes, 

 such as the herring or the giant sharks, and the baleen of the toothless 

 whales, all illustrate this means of food-getting. The effectiveness of 

 this apparatus is shown by the fact already stated that a whole new 

 world of minute plankton creatures, the nannoplankton, which had 

 escaped the finest silk nets, was discovered by examination of the 

 appendiculate sieve, and by the presence in the stomach of a single 

 herring of 60,895 small copepods. 21 



