970 CRUSTACEA 



of these larvae are so peculiar, and so entirely different from any of 

 those into which they are ultimately to be developed, that they were 

 considered as belonging to a distinct genus, Zoea, until their real 

 nature was first ascertained by Mr. J. Y. Thompson. Thus, in the 

 earliest state of Carcinus mcenas (small edible crab), we see the head 

 and thorax, which form the principal bulk of the body, included 

 within a large carapace or shield (fig. 722, 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 pro- 

 pulsion of the animal is chiefly effected. Its condition is hence 

 comparable, in almost all essential particulars, to that of Cyclops. 

 In the case of the lobster, prawn, and other ' macrurous ' species, 

 the metamorphosis chiefly consists in the separation of the loco- 

 motor 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 



FIG. 722. Metamorphosis of Carcinus mcenas: A, first or Zoea 

 stage ; B, second or Megalopa stage ; C, third stage, in which 

 it begins to assume the adult form ; D, perfect form. 



body) for the latter ; while the abdominal segments increase in size 

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

 the crabs, or ' brachyurous ' species, on the other hand, the altera- 

 tion 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 sup- 

 posed 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, which are developed into chelce or 

 pincers ; and the little creature entirely loses the active swimming 

 habits which it originally possessed, and takes on the mode of life 

 peculiar to the adult. 1 



In collecting minute Crustacea the ring-net should be used for 



1 On the metamorphoses of Crustacea and Cirripedia, see especially the T T nter- 

 suchungen uber Crustaceen of Professor Glaus, Vienna, 1876. A number of 



