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METAMORPHOSIS OF DECAPOD CRUSTACEA. 



genus, Zoea, until their real nature was first ascertained by Mr. 

 J. V. Thompson. Thus, in the earliest state of Carcinus manas 

 (small edible Crab), we see the head and thorax, which form the 



Metamorphosis of Carcinus mamas : — a, First or Zoea stage ; 

 B, Second or Megalopa stage ; c, Third stage, in which it begins 

 to assume the adult f orm ; d, Perfect form. 



principal bulk of the body, included within a large Carapace or 

 shield (Fig. 333, a) furnished with a long projecting spine, beneath 

 which the Fin-feet are put-forth : whilst the Abdominal segments, 

 narrowed and prolonged, carry at the end a flattened Tail-fin, by 

 the strokes of which upon the water, the propulsion of the animal 

 is chiefly effected. Its condition is hence comparable, in almost 

 all essential particulars, to that of Cyclops (§ 499). In the case 

 of the Lobster, Prawn, and other Macrourous species, the Meta- 

 morphosis chiefly consists in the separation of the Locomotive and 

 Respiratory organs ; true Legs being developed from the Thoracic 

 segments for the former, and true Gills (concealed within a special 

 chamber formed by an extension of the Carapace beneath the 

 body) for the latter ; and the Abdominal segments increase in size, 

 and become furnished with appendages (false feet) of their own. 

 In the Crabs, or Brachyourous species, on the other hand, the 

 alteration is much greater; for besides the change first noticed 

 in the Thoracic members and Respiratory organs, the Thoracic 

 region becomes much more developed at the expense of the 

 abdominal, as seen at b, in which stage the Larva is remarkable 

 for the large size of its Eyes, and hence received the name of 

 Megalopa when it was supposed to be a distinct type. In the 

 next stage, c, we find the Abdominal portion reduced to an almost 

 rudimentary condition, and bent under the body ; the Thoracic 

 limbs are more completely adapted for walking, save the first pair, 



