Page:Popular Science Monthly Volume 16.djvu/666

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by naturalists as equal units, formed of the same parts, having each a real individuality. The name Somites, which has been given them, shows the tendency to consider them as true elementary animals associated in colonies (Figs. 13 and 14). The power possessed by the segments of certain worms to individualize themselves and form new colonies is strong evidence in favor of this view. Polymorphism and the concentration of parts explain how a Peripatus or a Myriapod can

PSM V16 D666 Lobster with separated somites.jpg
Fig. 14.—Lobster with the Somites separated from each other, the Appendages being all removed except the Terminal Swimmerets. ca, carapace; t, telson; 2, third abdominal somite with its appendages.

become a spider or an insect, how different Crustacea arise from a common stem, how from another form of colony have arisen all the Annelida, It has been often said that Echinoderms, Star-fishes, Ophiurans, were only colonies united by the head (Fig. 15). They are, at least, all colonies, but of a special nature.

Can we say as much of the Mollusca and Vertebrata, all the parts of which are so closely united, and which are the giants of creation? Are there simple forms of association which can explain the marvelous organization of these superior types of creation—as we have explained the Siphoniferæ, Coral Polyps, Echinoderms, and Arthropoda?