62 HISTORY OF CRUSTACEA. CHAP. VII. 



kept, as in Mysis, in constant whirling motion. The 

 heart acquires new fissures, and interior muscular 

 trabeculae. 



During the Mysis-period, the auditory organs in the 

 basal joint of the anterior antennas are formed ; the 

 inner branches of the first three pairs of feet are deve- 

 loped into chelae and the two hinder pairs into ambula- 

 tory feet; palpi sprout from the mandibles, branchiae 

 on the thorax, and natatory feet on the abdomen. The 

 spine on the labrum becomes reduced in size. In this 

 way the animal gradually approaches the Prawn-form, 

 in which the median eye has become indistinct, the 

 spine of the labrum, and the outer branches of the 

 cheliferous and ambulatory feet have been lost, the 

 mandibular palpi and the abdominal feet have acquired 

 distinct joints and setae, and the branchiae come into 

 action. 



In another Prawn, the various larval states of which 

 may be easily recognised as belonging to the same 

 series by the presence of a dark-yellow, sharply-defined 

 spot surrounding the median eye, the youngest Zoe'a 

 (fig. 32), probably produced from the Nauplius, agrees 

 in all essential particulars with the species just de- 

 scribed ; its further development is, however, very dif- 

 ferent, especially in that neither the feet of the middle, 

 nor those of the hind-body are formed simultaneously, 

 and that a stage of development comparable to Mysis in 

 the number and structure of the limbs does not occur. 



Traces of the outer .maxillipedes make their appear- 

 ance betimes. Then feet appear upon four segments 



