THE WONDERS OF LIFE 



among the annelids, and the crawHng crabs {cirripedia), 

 among the Crustacea. All these stationary metazoa 

 move freely in their youth, and swim about in the water 

 as gastruke, or in some other larva form. They have 

 taken only gradually to stationary habits, and have been 

 considerably modified, and often greatly degenerated, in 

 consequence; for instance, in the loss of the higher 

 sense-organs, the bones, and even of the whole head. 

 Arnold Lang has shown this very clearly in his excellent 

 work on the influence of stationary life on animals. 

 The study of these retrogressive metamorphoses is very 

 important for the theory of progressive heredity and 

 selection; it also shows the great value of free locomo- 

 tion for the higher sensitive and intellectual develop- 

 ment of the animals and man. 



In many of the lower aquatic metazoa the surface of 

 the body is covered with vibratory epithelium — that is 

 to say, with a layer of skin-cells which bear either one 

 long whip (fiagellum) or several short lashes (cilia). 

 Flagellated epithelium is especially found in the cnidaria 

 and platodes ; ciliated epithelium mostly in the vermalia 

 and mollusca. As the lashing motion of these hairlike 

 processes brings a constant stream of fresh water to the 

 surface of the body, they first of all effect respiration 

 through the skin. But in many of the smaller metazoa 

 they also serve the purpose of locomotion, as in the 

 gastraeads, the turbellaria, the rotifera, the nemertina, 

 and the young larvae of many other metazoa. The 

 vibratory apparatus reaches its highest development in 

 the ctenophora. The extremely delicate and soft body of 

 these gherkin-shaped cnidaria swims slowly in the water 

 by means of the strokes of thousands of tiny oar-blades. 

 They are arranged in eight longitudinal rows which 

 stretch from the mouth to the opposite pole. Each oar- 

 blade consists of the long hair-lashes of a group of 

 epithelial cells glued together. 



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