Page:EB1911 - Volume 18.djvu/244

 to enable the important and far-reaching transformation to be accomplished. It is clear that the two kinds of metamorphosis only differ in degree and that no line can be drawn between them.

In the Crustacea, as has already been, pointed out, many authors apply the term metamorphosis to the whole larval development, which consists of a series of changes leading to the adult form. But this is in our opinion an incorrect use of the word. The typical larval development of a Crustacean consists of a series of small metamorphoses. At each moult new organs which have been developed since the preceding moult become manifest and some of them functional. For instance, the prawn Penaeus leaves the egg as a nauplius larva (fig. 3, A). It issues from the first moult as a metanauplius which has a forked tail, a beginning of the cephalo-thoracic shield, and a large helmet-shaped upper lip. It also possesses stump-like rudiments of the maxillae and two anterior pairs of maxillipeds. After the next moult it is known as a protozoaea (fig. 3, B), in which a cephalo-thoracic shield is well developed, the posterior part of the body is prolonged into a tail, in the anterior part of which the thoracic segments are obscurely indicated, and the four pairs of stump-like rudiments have become functional appendages [fig. 3, B (4), (5), (6), (7)]. This passes into a later protozoaea stage (C) in which the rudiments of the compound eyes and of the abdominal segments are visible beneath the cuticle and in which certain functional changes (jointing, &c.) have appeared in the limbs. This is succeeded by the' zoaea stage (fig. 3, D), characterized by the stalked and functional condition of the eyes, the increased size of the abdominal segments, and the appearance of appendages on the sixth of them, the increase of size in the third pair of maxillipeds (8) which had appeared as small rudiments' in the preceding stage, and the appearance of the five pairs of posterior thoracic limbs as small biramous appendages. The zoaea stage is followed by the mysis stage (fig. 3, E) in which the thoracic feet are biramous, as in Mysis. From this the adult form proceeds. The transformation is more gradual than would be gathered from this short description, because moults occur during the later stages from each of which the larva comes with some slight transformation. In the life-history of a typical Cirripede there may be said to be two distinct metamorphoses, with gradual developmental stages taking place. between them. The animal is hatched as a nauplius. This undergoes a series of moults during which increase in size and slight changes in form occur (fig. 4, A, B). At the last of them several organs characteristic of the second