The crab, on leaving the egg, enters the world not in the form of a Nauplius, but as a 'Zoea,' as it is called; this is shown in figs. 1 and 2. By the time the stage shown in figs. 3 and 4 is reached, he has attained the dignity of what is known among scientific men as the megalopa stage. What a nauplius looks like you will see in fig. 8.
Fig. 7.—Lobster.
Fig. 8.—Nauplius.
Fig. 9.—Swimming Foot of Crayfish, with the young ones attached.
This curious order of things is true, however, only of salt-water members of the crab-tribe. With certain near relatives of the crabs and lobsters which have taken up their residence in fresh water, a different order of things prevails, for here we find some trace of maternal care. Thus, in the fresh-water crayfish the young not only leave the egg in a much more advanced stage, but they are carefully carried about by the mother, until they have learned to shift for themselves, which they do in a very few days. During this time they cling to the swimming legs of the parent by means of their pincers. When all is quiet they drop off one by one, and crawl about to gather experience and food. But at the least sign of danger the mother appears to give some note of warning, and in a moment they have scuttled back, and fastened hold of her skirts, so to speak; then, if need be, she hurries off to a place of safety. At this time these little crayfish are very tiny indeed; but to get an idea of what they look like, and how they hold on, look at fig. 9, which gives a picture of the swimming foot of a mother crayfish, and two of her youngsters hanging on to it.
It would seem that this great care is necessary, because in the swift-running streams where these creatures generally live, their young, if uncared for during their early days, would be swept away by the tide and carried out to sea, where they would speedily die.