Fig. 3.—Last stage of Crab's Infancy: back view.
Fig. 4.—Side view of Fig. 3.
A little later he assumes a form which is certainly fantastic, for from the top of the shell (as you will see in fig. 2) there grows out a long, curved spine, while the legs have taken a shape suggesting jointed paint-brushes. Later still the eyes grow out from the head and are supported on short stalks, the legs and pincers appear, and lastly the long tail curls up, till at last it grows into a curious three-cornered shield and is carried tucked away under the body. As soon as this takes place he becomes at once the crab with which we are familiar, and changes no more except in size.
The strange flap hinged to the hinder edge of the body of the adult crab, and held close to its under surface, really answers to the long body of the lobster. To make this clear, look at the series of figures 3 to 6: fig. 3 shows the back view of the last stage of the crab's infancy, and fig. 4 a side view of the same, where you will note that the tail is already beginning to curl up. In fig. 5 you have the under side of the full-grown crab with all that remains of the hinder part of the body and the tail in the position in which it is carried during life, and in fig. 6 the upper side with the tail showing as if it were unfolded. If you compare figs. 5 and 6 with fig. 7 (a lobster), you can see that the hinder part of the body of the crab, with the tail unfolded, really does answer to that portion of the body of the lobster which lies behind the last pair of legs.
Fig. 5.—Full-grown Crab, under side, showing tail curled up.
Fig. 6.—Full-grown Crab, upper side, with tail unfolded.
But there are some relatives of the crab which come into the world still earlier, and these in the early stages have a still more un-crab-like shape which is known as the nauplius stage of growth.