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Part III — Twenty-third Annual Report 



with the pleopods of the megalops. The telson of the a IV." stage, 

 which was drawn, had no median spine on the hind border ; in this 

 stage a median spine is usually present. 



Stage " IV." attracts attention from the fact that in general shape and 

 large size it resembles a megalops. The use of the pleopods for 

 swimming give it the characteristic megalops appearance. It swims 

 with the chela? stretched straight out in front of it. It may be 

 regarded either as a backward megalops, or as a precociously developed 

 zoea. From the point of view of the former, the antennae, which are so 

 prominently employed by the megalops, have developed more slowly 

 than the pleopods. We have, in fact, a megalops which has carried over 

 certain zoea characters, viz. antennules, antennae, the purely maxillipede 

 form of the pereiopods, and the abdominal hooks. There are other cases 

 in which minor zoea characters are carried over and exhibited in the 

 megalops ; they will be referred to later. If the second view is adopted, 

 we are led to the interesting conclusion that an organ may by precocious 

 development become functional in a stage which is normally without it. 



Might not an unusually rapid growth of the zoea in size necessitate the 

 earlier provision of swimming organs to assist the exopodites which were 

 sufficient in the smaller stages 1 Or might a lower salinity react by 

 stimulating the development of greater swimming power? The zoea has 

 attained to the body of a megalops, and the result is the provision of the 

 means of moving it about. 



Boas describes considerable difference in structure between the larvae 

 and adults of the fresh-water and sea-water forms of Palaemonetes 

 varians. The larva of the former is larger than that of the latter. 



The megalops stage is illustrated by several figures. Fig. 72, pi. iv.. 

 shows the lobster in this stage. The pereiopods are represented by figs, 

 60, 61, 62, and 58, pi. iii., while the abdomen and telson are shown in 

 figs. 57 and 63, pi. iii. 



The exopodites of the pereiopods are present, and setose, though very 

 much reduced ; but variations in the exopodites are common. In the 

 stage following the megalops, viz. the first young stage, the exopodites 

 are reduced to little processes (vide figs. 59, 65, 70, pi. iv.). 



While dissecting a megalops the first pereiopods broke off at the 

 junction between the basipodite and the ischiopodite. These joints, so 

 far as could be made out, were fixed, as they are in the adult. This is 

 the fracture plane of Fredericq. The broken limb showed a clean but 

 not very regular break (fig. 58, pi. iii.). The muscles in the ischiopodite 

 run right down and terminate at the proximal end of that segment. The 

 muscles of the exopodite may have something to do in effecting the 

 fracture. 



The pleopods are similar to those of Stage " IV." (fig. 51, pi. ii.). The 

 setae have long, stiff cila, and resemble generally the setae on the pleopod 

 of the megalops of Crangon vulgaris. 



The telson of the megalops had a median spine on the hind border. 

 This spine is usually absent ; it is a zoea character. 



The chela resembles that of the first young stage (fig. 65), but the 

 tubercles on the meropodite are a little less prominent. 



The first young stage resembles much the megalops (vide fig. 70, pi. iv.), 

 but is usually larger. The exopodites of the thoracic limbs are small 

 processes, no longer setose. The antennae are longer than in the megalops. 

 The pleopods are similar to those of the megalops. The rostrum is 

 bifurcate. On the whole, the lobster in the first young stage resembles 

 much in its habits the lobster in the megalops stage. It does not appear 

 to swim quite so much. 



The first pereiopod of this stage is figured in figs. 65 and 59, pi. iii 



