Many of the crustaceans are so small that but few persons ever see them. Such is Cyclops (Fig. 137), a minute creature seen distinctly only under a microscope, yet swimming in fresh water and readily recognized by its egg pouches, one on either side of the tail. The eggs hatch out into singular little objects, having very little resemblance to the parent. The Cyclops and others are very tenacious of life. When pools and streams dry up and remain so for months, they lie dormant, coming to life again with the return of the water. Many of this group are parasites upon fishes, as the Lernæidæ (Fig. 138), which appear like streamers on the sides of carp and other fishes. These parasites, deeply embedded, live upon the fish. [Illustration: FIG. 137.--Water fleas: 1, Cyclops, showing egg pouches; 2, Cypris; 3, Daphnia.] [Illustration: FIG. 138.--A parasite of a fresh-water fish (_Cyprinæ_): 1, larva, as it leaves the egg; 2, larva, more advanced; 3, adult female, showing the egg sacs. (Nordmann.)] Some of these minute crustaceans are almost exact in their resemblance to shells, as Estheria which has a bivalve shell. But perhaps the most remarkable creature is Artemis, the brine shrimp (Fig. 139), which lives in brine that would be deadly to almost any other animal. A strange experiment has been made with this little creature; thus if the brine is very strong its form resembles _a_, but if the brine is diluted, it changes to _b_, a very different animal, so different that it has been given another name. Many shrimps seem to prefer extreme cold. The Apus (Fig. 140), withstands freezing, and hatches readily in the icy water of the far north. This little creature has forty-seven segments and one hundred and twenty legs. The fairy shrimp is a dainty and beautiful crustacean with a marvelous array of leaflike feet which also serve as breathing organs. [Illustration: FIG. 139.--Brine shrimps: _a_, Branchipus; _b_, Artemis.] [Illustration: FIG. 140.--Apus.] In the summer, while strolling alongshore, one may find that every piece of seaweed or rock when turned over affords concealment to myriads of "sand fleas" which belong to a group of crustaceans having fourteen feet. The sand fleas, true to their name, are remarkable jumpers, darting in all directions and looking very much like an ordinary flea (Fig. 141). They are valuable scavengers, eating all kinds of refuse matter. They have the most bizarre shapes, and many, as Arcturus, resemble twigs or pieces of seaweed, extremely difficult to see and doubtless owe their immunity from attack to this cause. This Arcturus (Fig. 142) is not only a remarkable mimic, but carries its young upon its back. Idotea is a common form about piers, while the little Gammarus may be caught with almost every haul of a very fine net. At times one known as Podocerus builds a singular nest for its better security, and one of the giants of the tribe has eyes so huge that they are made up of facets and entirely cover the head.