The larval form of the Common Lobster has already been described, and it will be noticed that the differences from the adult are much less than in the case of the Crab. From the fact that this larva has swimming exopodites on its legs, like the adult Mysidacea and Euphausiacea (formerly grouped together as "Schizopoda"), it is said to be in the "schizopod stage." The larva of the Norway Lobster (Nephrops norvegicus) is essentially of the same type, but the great development of the spines on the abdomen and of the forked telson gives it a striking appearance.
A very remarkable type of larva is found among the Spiny Lobsters and their allies (Scyllaridea). This larva, known by the name of phyllosoma ([Fig. 28]), is very broad, thin, and leaf-like, and quite transparent, so that some of the larger kinds were formerly known as "Glass Crabs." The thin oval carapace does not cover the whole of the thoracic region, which is disc-shaped, with four pairs of long slender legs, each with an exopodite. The abdomen is relatively small. The intermediate stages between the phyllosoma and the adult are still very imperfectly known. In tropical seas phyllosoma larvæ of large size are found, sometimes reaching two or three inches in length. The larva of the Common Spiny Lobster (Palinurus vulgaris), however, does not exceed half an inch in length.
Fig. 28—The Phyllosoma Larva of the Common Spiny Lobster (Palinurus vulgaris—see [Plate V].). Much enlarged. (After J. T. Cunningham.)
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The Shrimps and Prawns of the tribe Caridea are mostly hatched as zoëæ, and pass through a "schizopod" stage comparable to that of the Lobster, in which they swim by means of exopodites on the legs. Some of the Prawns belonging to the tribe Penæidea, however, have a still more remarkable metamorphosis, which is very important on account of the resemblance of the earlier stages to those of the lower Crustacea. Fritz Müller discovered in 1863 that Penæus is hatched from the egg as a Nauplius ([Fig. 29], A), a form of larva which was previously known among the Copepoda, Branchiopoda, and Cirripedes. The nauplius, unlike the larvæ which we have been considering, has an unsegmented body, and has only three pairs of limbs. The body is pear-shaped in outline, and near the front end is seen the median eye, sometimes called, from its presence in this type of larva, the "nauplius-eye"; the paired eyes are not yet developed. The three pairs of limbs are shown by their later development to be the antennules, antennæ, and mandibles; the first pair are unbranched, the second and third divided into exopodite and endopodite. It is interesting to notice that the antennæ and mandibles, which in the adult animal are so widely different that it is difficult to trace any resemblance between them, are in the nauplius almost identical in form. Further, the antennæ, instead of being placed in front of the mouth as in the adult, lie on either side of it, and each has at its base a hooked spine which projects inwards and serves for seizing particles of food and passing them into the mouth; the antennæ of the nauplius, in fact, serve as jaws, while it is only later that the mandibles take on this function.
Fig. 29—Larval Stages of the Prawn—Penæus ( see [Plate IV].). × 45. (After F. Müller.)