The Lobster, like all Crustacea, undergoes a series of moults as the result of increase in size, shedding the whole of the external integument in one piece. This is accomplished by a split taking place on the dorsal surface at the junction of thorax and abdomen; through the slit so formed the Lobster retracts first his thorax with all the limbs, and then his abdomen. When first issuing from the old shell the animal’s integument is soft and pulpy, but the increase in size of the body is already manifest; this increase per moult, which is approximately the same in young and adult animals, varies from 13 to 15 per cent of the animal’s length. According to this computation, a Lobster 2 inches long has moulted fourteen times, 5 inches twenty times, and 10 inches twenty-five times, and it may be roughly estimated that a 10–inch Lobster is four years old. Young Lobsters probably moult twice a year, and so do adult males, but females only moult once a year soon after the young are hatched out.
The process of moulting or ecdysis is an exceedingly dangerous one to the Lobster and to Crustacea in general, and is very frequently fatal. There is, first of all, the danger of the act not being accomplished skilfully, when death always ensues. The Lobster remains soft and unprotected for about six weeks after the ecdysis, and is very apt to fall a prey to the predaceous fish, such as Sharks, Skates, Cod, etc., which feed upon it. There are, however, some peculiar adaptations connected with the process which are of interest. In order to facilitate the ecdysis, areas of absorption are formed upon the dorsal and ventral surfaces of the carapace, on the narrower parts of the chelipedes, and at other places; in these areas the calcium carbonate is absorbed, and the old shell becomes elastic and thin, so as to allow a more easy escape for the moulting Lobster. It has been noticed that while this is taking place large concretions of calcium carbonate are formed at the sides of the stomach, known as “gastroliths,” which perhaps represent the waste lime that has been abstracted from the areas of absorption. After moulting the Lobster is in great need of lime for stiffening his shell, and it has been noticed that on these occasions he is very greedy of this substance, even devouring his own cast-off skin.
The male Lobster is especially prized on account of his larger chelae, but in both sexes the chelipedes are differentiated into a smaller cutting pincer and a larger crushing one. Lobsters may be right or left handed, with the large crushing claw on the right or left hand, and sometimes specimens occur with the smaller cutting pincers on both chelipedes, and very rarely, indeed, with crushing claws on both sides. Crustacea very commonly have the power of casting off a limb if they are seized by it or if it is injured, and of regenerating a new one. In the Lobster a so-called breaking joint is situated on each leg at the suture between the fused second and third segments; a membrane being pushed inwards from the skin, which not only serves to form a weak joint where rupture may easily take place, but also to stop excessive bleeding after rupture. In the newly-hatched larvae there is a normal joint between the second and third segments; and autotomy, or the voluntary throwing away of a limb, never occurs until the fourth larval stage, when the breaking joint is formed. Autotomy is a reflex act under the control of the segmental ganglion; if a Crab or Lobster be anæsthetised, and then a limb be injured or broken off below the breaking joint, the animal forgets to throw the injured leg or stump off at the breaking joint, a proceeding which always occurs under normal conditions. The regeneration of a limb starts from a papilla which grows out of the breaking joint, and after a number of moults acquires the specific form of the limb that has been lost. A number of interesting observations have been made upon the regeneration of the limbs in Crustacea. It was in the Hermit-crab that Morgan[[129]] proved that regeneration and the liability to injury do not always run parallel, as Weismann held they should, since the rudimentary posterior thoracic limbs, which are never injured in nature, can regenerate when artificially removed as easily as any others. Przibram[[130]] has shown that in the shrimp Alpheus, whose chelipedes are highly asymmetrical, if the large one be cut off, the small one immediately begins to grow and to take on the form of the large one, while the regenerated limb is formed as the small variety. This remarkable inversion in the symmetry of the animal clearly ensures that, if the large chela is injured and thrown away, the least amount of time is wasted in providing the shrimp with a new large claw.
To return to the Lobster; for the majority of the individuals there is a definite breeding season, viz. July and August, but a certain proportion breed earlier or later. A female begins to “berry” at about eight inches in length, and to produce more and more eggs up to about eighteen inches, when as many as 160,000 eggs are produced at a time; after this there is a decline in numbers. A female normally breeds only once in two years. Strict laws are enforced forbidding the sale of Lobsters and Crabs “in berry” in both England and America. The period of incubation, during which the developing eggs are attached to the swimmerets of the female, lasts about ten or eleven months, so that the larvae are hatched out approximately in the following June. On hatching, the larva, which measures about one-third of an inch, and is in the Mysis stage (i.e. it possesses all the thoracic limbs in a biramous condition, but is without the abdominal limbs), swims at first on the surface. After five or six months of this life, during which the abdominal pleopods are added from before backwards, it sinks to the bottom, loses the exopodites of the thoracic limbs, and is converted into the young Lobster, measuring about half an inch in length. The little Lobster starts in deepish water, and gradually crawls towards the shore; here it passes its adolescence, but on coming to maturity it migrates out again into the deep water.
Fam. 2. Astacidae.—In this family, which includes all the European and North American Crayfishes, Astacus (Potamobius) and Cambarus, the podobranchs are united with the epipodites, the last thoracic segment is free, there is only one pleurobranch or none at all, the gills have a central lamina, but the filaments are without terminal hooks, and the endopodites of the first two pairs of abdominal appendages in the male serve as copulatory organs. For the distribution, etc., of these forms see p. [213].
Fam. 3. Parastacidae.—This family includes the Crayfishes of the Southern Hemisphere, viz. Parastacus from South America, Astacopsis and Engaeus from Australia, Paranephrops from New Zealand, and Astacoides from Madagascar. These genera agree with the Potamobiidae in the union of the podobranchs with the epipodites, and in the free condition of the last thoracic segment, but there are generally four pleurobranchs, the gills are without a lamina, the filaments have terminal hooks, and there are no sexual appendages in the male. For distribution, etc., see also p. 213.
The larval development in the Crayfishes is still more abbreviated than in the Lobsters, the Mysis stage being passed through within the egg-membranes. The young, when they hatch out, are furnished with hooks upon the chelipedes, by which they anchor themselves to the pleopods of the mother.
Tribe 2. Eryonidea.
Fig. [105].—Willemoesia inornata, × ⅓. (From a figure prepared for Professor Weldon.)