Lobsters, especially, have an amazing fecundity, and yield an immense number of eggs, each female producing from 12,000 to 20,000 in the season. The crab is also very prolific. These eggs are, in the lobster, arranged in packets, which are attached to the lower surface of the tail, to which they are connected by a viscous substance. The manner in which the female lobster disposes of her burden is curious and interesting. Whether she bends or stands erect she is able to hold it obscurely or expose it to the light. Sometimes, according to Coste, the eggs are left immovable, or simply submerged; at others they are subjected to successive washings by gently agitating the false claw which shelters them from right to left. When first exuded from the ovary the eggs are very small, but they seem to increase during the time they are borne about under the tail, and before they are committed to the sand or water they have attained the size of small shot. The evolution of the germ is in progress during six months. At the moment of exclusion the female extends the tail, impresses upon the eggs an oscillating motion, in order to destroy the shell and scatter the larvæ, delivering herself in two or three days of her entire burden (Coste). "As the young lie enclosed within the membrane of the egg," says Couch, "the claws are folded on each other, and the tail is flexed on them as far as the margin of the shield. The dorsal spine is bent backwards, and lies in contact with the dorsal shield, for the young when it escapes from the egg is quite soft; but it rapidly hardens and solidifies by the deposition of calcareous matter on what may be called its skin."
Fig. 333. Palinurus vulgaris. a, left outward jaw-foot.
As soon as born, the young Crustaceans withdraw from the mother and ascend to the surface of the water, in order to gain the open sea. They swim in a circle; but this pelagic life is not of long duration; they quit it after their fourth moult, which takes place between the thirtieth and fortieth day, at which time they lose the transitory organs of natation which they have hitherto possessed. After this they are no longer able to maintain themselves on the surface, but drop to the bottom. Henceforth they are condemned to remain there, and such walking as they can exercise becomes their habitual mode of progression. As they increase in size they gradually approach the shore, which they had for the moment abandoned, and return to the places inhabited by the parent Crustaceans.
Fig. 334. Portunus variegatus, male.
a, external antenna; b, external jaw-foot; c, tail or abdomen.
The form of the larvæ differs so much from that of the adult, that it would be difficult, except on the clearest evidence, to determine the species from which they proceed. Former naturalists considered the embryo cray-fish (Palinurus) to belong to a distinct genera, which they designated Phyllosoma. It is now known, however, that these are the young of the higher forms of Crustaceans undergoing metamorphosis. In the various forms of Macroura the metamorphosis is less decided than in the Brachyura. In the fresh-water cray-fish no change whatever takes place. Dissatisfied with the uncertainty of former experiments, Mr. Couch undertook a series of observations, which are recorded in the proceedings of the Cornwall Polytechnic Society, in which he established the fact that metamorphosis takes place in the following genera: Cancer, Xanthò, Plumnus, Carcinus, Portunus, Maja, Galathea, Hornarus, and Palinurus. "Metamorphosis has been demonstrated," says Dr. Bell, "in no less than seventeen genera of the Brachyurous order of Decapoda, in which it is most decided and obvious; in Leptopodia, Majacea, Cancer, Portunidæ, Pinnoteres, and Grapsus. In the Anomourous order it is seen in the Pagurus, Porcellana, and Galathea; and in the Macrouran order in Homarus, Palinurus, Palæmon, and Crangon."
The swimming of these creatures is produced by flexions and expansions of the tail, and by repeated beating motions of the claws, the tail acting as a sort of vibratile oar, aided by which they maintain themselves in the water and facilitate their progress. As the shell becomes more solid they get less active, and finally return to the bottom to cast their shell and assume a new form.
According to the observations of M. Coste, the young lobster casts its shell from eight to ten times in the first year, from five to seven in the second, three to four times the third, and two or three times the fourth year. In the fifth year they attain the adult state. Whence it follows, that the small lobsters served at our tables have changed their calcareous vestment something like twenty-one times, and are now clothed in their twenty-second habit.