of the Common Shore- Grab, 203 



we perceive that the progress made is not by any- 

 sudden metamorphosis, but by a series of moultings similar 

 to those which take place in the adult ; and that with each 

 successive moult there is a corresponding degree of progress 

 in its development. But the amount of change at each moult 

 is so little, that it gives to the animal but a very small degree 

 of difference in its general appearance ; and it is only by a 

 comparison of the earliest form with the last, and that without 

 any consideration of the intermediate stages in its growth, that 

 the idea of a true metamorphosis in Decapod Crustacea has 

 existed. There are six or seven well-marked stages or forms 

 that the growing animal passes through in its progress to 

 maturity, and each of these is linked to the preceding as well 

 as to that which follows, by a succession of changes that 

 are but just appreciable." And again (p. 596), "Successive 

 moults rob the young animal soon of the frontal spine. Con- 

 temporary with its decreasing importance, t\\e, pleon becomes 

 gradually folded nearer and nearer, until it is closely com- 

 pressed against the inferior surface of the pereiony 



This appears perfectly true of the development of the Zoea 

 into the Megalopa^ and up to a certain point the Megalopa 

 approaches nearer and nearer the adult form with each moult. 

 The last stage which can be called a Megalopa is shown in 

 fig. 1. Here the frontal spine is very much reduced in size, 

 and the dorsal one lias disappeared altogether. Th.Q, pleon is 

 very much reduced also, and in its natural position is sickle- 

 shaped, showing that ultimately it will be curled under the 

 pereion. There is also a ridge forming on each side of the 

 carapace, which is to be pushed forward and form the lateral 

 toothed margin of the adult. The next moult is, however, a 

 comparatively sudden change to the true Brachyuran form. 

 The frontal spine is lost, the pleon is now curved under the 

 pereion, and the ridge which was seen in the hepatic region 

 of the carapace now forms the lateral toothed margin (fig. 2). 

 All the development up to this stage has been gradual and 

 preparatory, but now the last traces of larval form are thrown 

 off all at once, and, generally speaking, it may be said that 

 afterwards the animal only grows larger. Of course the 

 carapace is not yet of the shape it will ultimately have ; but 

 what I wish to enforce is, that at this particular moult the 

 larval characters are lost and the adult ones assumed. 



Dr. Brooks in his * Invertebrate Zoology,' gives in figs. 110 

 and 117 drawings of the Megalopa of Gallinectes hastatus^ 

 and of the young crab which hatches from it. These figures 

 agree in every respect with what has been here stated as to 

 Carcinus mcenas. 



16* 



