LOWER ORGANISMS 



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transplanted to the waters of northern and western Europe. Adhering to the 

 sides of ships they have been carried to the coasts of the Atlantic, the North Sea, 

 and the Baltic, whence they have ascended the rivers; and they have thus made 

 their way right across Germany, up the Rhine into the Main, by means of the 

 Main Canal into the upper course of the Danube, and down the Danube to their 

 original home in the Black Sea. 



Lower Leaving the molluscs, we may 



Organisms, briefly notice a few representatives 

 of the multitude of lower forms of life. Of the 

 Bryozoa the commonest is Plumatella repens,& 

 foot or more long, consisting of club-shaped 

 tubes, with about sixty tentacles, arranged in 

 the shape of a horse-shoe, bearing single animals 

 measuring a quarter of an inch. Of the 

 Ccelenterata, which are represented in fresh 

 water by the hydra and a few other forms, 

 the chief characteristics are the radiate structure 

 and the absence of alimentary canal, circulatory 

 system, and respiratory apparatus, the animal 

 being merely a system of cavities, in free com- 

 munication with one another. 



The most common fresh-water species are 

 the green Hydra viridis and the brown H. 

 fusca. A hydra is essentially a tube, consisting 

 of two layers with a crown of from six to ten 

 hollow tentacles round the mouth ; and it can be 

 cut into pieces, and each piece will grow into a 

 complete animal, or it can be turned inside out 

 and go on living as if nothing special had 

 occurred. It fixes itself on aquatic plants, often 

 to the lower side of a duckweed leaf, and feeds 

 on small organisms sufficiently definite in struc- 

 ture to irritate its stinging cells into action. 



In many inland waters occur sponges, 

 representing a still lower subkingdom, that of 

 the Porifera. These assume an endless variety 

 of shapes, but in all cases the water passes 

 through the fine pores, and is driven out through 

 the larger ones. The two Em-opean species are 

 the pond-sponge (SpongiUa lacustris) and the river-sponge (8. fiuviatilis). 



Lowest of all among the subkingdoms of the animal series are the Protozoa, 

 which are microscopic and of very simple structure. Among these, the trumpet- 

 animalcule (Stentor polymorphus) is -^ of an inch in length, and covered with cilia 

 all along the upper surface, but most abundantly at the edge of the trumpet-like fore- 

 end : and with these cilia it whirls the food into its body. The tongue-animalcule 

 (Aciveta linguifera) has a body which protrudes like a tongue from a fixed, stalk - 



THE TRUSU'ET-ANIMALCULE. 



