216 THE MICROSCOPE AND ITS REVELATIONS. 



metamorphosis, whether they are afterwards to belong to the macrourons 

 (long-tailed) or to the brachyourous (short-tailed) division of the group; 

 and the forms 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. V. Thompson. Thus, in the 

 earliest state of Carcinus mmnas (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. 413, 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 essen- 

 tial particulars, to that of Cyclops ( 605). In the case of the Lobster, 

 Prawn, and other f macrourous ' species, the metamorphosis chiefly con- 

 sists 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 cara- 

 pace beneath the body) for the latter; while the abdominal segments 



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. 



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 dis- 

 tinct 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 chela 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 



615. In collecting minute Crustacea, the King-net should be used for 

 the fresh-water species, and the Tow-net for the marine. In localities 

 favorable for the latter, the same ' gathering ' will often contain multi- 



1 On the Metamorphosis of Crustacea and Cirripedia, see especially the 

 recent " Untersuchungen liber Crustaceen" nf Prof. Glaus; Vienna, 1876. 



