The Arthropods are an extraordinarily successful group. A multitude of forms of Crustaceans populate the waters, and they are excelled in numbers and variety only by the insects upon land. While the individual size appears to be somewhat strictly limited, probably by the nature of the respiratory and blood systems, many types show exceedingly high development in various directions--in intelligence, in social and parental instincts, etc. The insects are of course to be regarded as the highest Articulata, and have, like the highest vertebrates, the mammals and birds, almost completely forsaken the water for the dry land and the air. An interesting member of the Articulata, from the standpoint of the Evolution theory, is the Peripatus, shown with a ringed worm on Fig. 52 (3). It gains its interest for us from the fact that, while classed as an Arthropod, it stands very nearly half-way between the ringed worms (Fig. 52 (1)) and the true Arthropoda, and thus forms a solitary link between the two types. In Fig. 52 (2) and Figs. 53 to 58 are shown a number of types of Arthropods. [Illustration: FIG. 53.--Arthropods--The Lobster.] [Illustration: FIG. 54.--Arthropods--Scorpion. Photo: Leonard Bastin.] [Illustration: FIG. 55.--Arthropods (insects)--Stag-horn beetle. Photo: Harold Bastin.] [Illustration: FIG. 56.--Arthropods (insects)--Dragon-fly. Photo: Harold Bastin.] [Illustration: FIG. 57.--Arthropods (insects)--Mantis. Photo: Harold Bastin.] [Illustration: FIG. 58.--Arthropods (insects)--Swallow-tail butterfly and larva. Photos: Harold Bastin.] [Illustration: FIG. 59.--Balanoglossus.]