ASTACIDEA. 



277 



Only in a later young form, which has sunk to the bottom, does the 

 asymmetry of the pincers become apparent ; the first abdominal limbs 

 which, to begin with, are similar in form in the two sexes, appear 

 at this same time. At a still later stage sexual differentiation 

 is attained. 



The metamorphosis of Nephrops norvegicus, a single stage of which was made 

 known long ago by Claus (No. 113), and which has more recently been 

 completely described by G. 0. Saks (No. 149), agrees in all its details with 

 that of Homarus. Here, as in the latter, the four stages described above can 

 be distinguished, being similarly characterised in both forms. The larva of 

 Nephrops, however (Fig. 127), has a peculiar appearance, being distinguished by 

 the spines on its abdomen. On the third abdominal segment there is a email 

 dorsal spine, on the fourth and fifth a much larger one, while the sixth abdominal 

 segment has a pair of long backwardly directed spines. The telson which, in the 



Fig. 127.— Later Mysis stage of Nephrops norvegicus (after Sars). a : , first antenna ; a", second 

 antenna; mf", third maxillipede ; p^-p 5 , first five ambulatory limbs; p!2, pleopod of the 

 second abdominal segment ; pi 5 , pleopod of the fifth abdominal segment ; r, rostrum. 



first and second stages, Avas not distinctly marked off from the sixth abdominal 

 segment, runs out into two very large movable spinous processes which are 

 retained even in the third stage, together with the lateral plates of the caudal 

 fin (sixth pair of pleopoda), and only disappear in the fourth stage, in which 

 the telson is transformed into the middle plate of that fin. 



The young of Astaeus, when hatched, are distinguished only in unessential 

 points from the adult. The cephalo-thorax is swollen by the mass of nutritive 

 yolk beneath it, and the rostrum is curved downwards between the eyes. The 

 first pair of abdominal limbs is still undeveloped. The pleopoda of the sixth 

 segment have also not attained free development. The telson has a peculiar 

 oval shape. In other respects the young of Astaeus, which still remain for some 

 time hanging to the abdominal limbs of the mother and are protected by her, 

 entirely resemble the adult. Metamorphosis here, as in so many fresh-wafer 

 forms, has entirely disappeared. 



The young of Cambarus (W. Faxon, No. 127) closely resemble in shape the 

 above forms, and, like those of Astacus, recall in certain respects the Paraslaci(/<>e. 

 The first and sixth pairs of pleopoda are still wanting, and the transverse 

 segmentation in the region of the telson-plate has not yet developed. The 



