OMNIVOROUS MOLLUSCS, INSECTS, AND CRUSTACEANS 255 



nothing of small particles of organic matter which may be present 

 in the surrounding water. Behind them come at least four pairs 

 of forked limbs, which, by their regular forward-and-backward 

 swing, assist in locomotion, and also sweep edible matters within 

 reach of the jaws. Dr. Vosseler (in Zacharias' The Animal- and 

 Plant-Life of Fresh Water) makes the following interesting re- 

 marks as to the feeding habits of the free -swimming members 

 of this group of Crustacea: " The food of all free-living Cope- 

 pods consists of small particles of animal and vegetable matter 

 which are found on the surface of the bottom-mud, on the stems 

 of plants, &c. Protozoa and microscopic algae appear to afford 

 the chief supply. As these are always present together, it is 

 difficult to make out whether these animals are actually omni- 

 vorous, or whether one or other kind of food is taken up acci- 

 dentally. According to some observations, the mother devours 

 her own children, and this cannibal behaviour suggests that the 

 Copepods may be purely carnivorous. It is very amusing to 

 watch these animals searching for food. If the vessel in which 

 they have been placed contains plants, they browse among the 

 debris covering the surface of these, picking their way in a 

 methodical manner. When the stomach is sufficiently filled, the 

 animal seeks once more the society of its comrades." 



Another widely-distributed group of the Lower Crustacea is 

 that of the OSTRACODS (see vol. i, p. 419), which includes a large 

 number of species of small marine and freshwater forms, living 

 on the surface of the mud, &c., at the bottom, or swimming among 

 water-plants. They are enclosed in a bivalve shell, almost like 

 that of a freshwater mussel, and possess, like Copepods, two 

 powerful pairs of feelers, serving not merely as sense organs, but 

 also as means for effecting swimming or assisting progression 

 on a firm surface. There are but three pairs of jaws, and behind 

 these are situated two pairs of pointed legs, used both in loco- 

 motion and for scraping food forward to the mouth. This food 

 appears to be partly of vegetable, partly of animal nature. 



The last group of Crustacea to be mentioned is that of the 

 PHYLLOPODS, distinguished by their flattened leaf-like limbs, and 

 including a comparatively small number of species, which are 

 mostly inhabitants of fresh water, or of salt lakes and lagoons. 

 There are two sub-groups, one embracing fairly large forms, of 

 which Apus has already been described as a type (vol. i, p. 421). 



