250 CRUSTACEA. 



the antennae also serve as oars, while, in the Zoaea of the Brachyura, 

 these limbs are kept in the background, and locomotion is carried on 

 exclusively by the two pairs of biramose maxillipedes in con- 

 junction with the movable abdomen. In many cases, the third 

 pair of maxillipedes has already begun to function. The limbs 

 which follow these may be present as sac-like, unjointed rudiments 

 in close contact with the body, but they never function in the Zoaea. 

 The pleopoda are still altogether wanting, except the sixth pair 

 (uropoda), which may, in individual cases, develop as early as this 

 stage. The spine-like process, which rises from the cephalo-thorax 

 and occurs typically in the Brachyuran Zoaea, was formerly considered 

 as a characteristic of this stage, and too great stress was laid upon 

 this point. A really important characteristic of this stage, however, 

 is the fact that the six posterior thoracic segments (commencing with 

 that carrying the third pair of maxillipedes) are generally rudimentary 

 and often difficult to recognise, while the abdominal segments are 

 very apparent on account of their large size and distinct boundaries. 

 The Zoaea stage indicates, in many Decapoda, the beginning of 

 metamorphosis, many leaving the egg in this condition. The 

 Protozoaea and Zoaea, in contrast to the Metanauplius, are dis- 

 tinguished by the gradual development of the paired stalked eye 

 which, as in Branchipus, first arises as a lateral outgrowth of the 

 cephalic region (cf. the Zoaea of Lucifer, Fig. 119 E, p. 260), 

 the eye-stalk developing very gradually. 



5. The Mysis stage (Fig. 120 A, p. 262, and Fig. 123 D, p. 269) 

 and Metazoaea stage (Fig. 133 B, p. 287). By the development of 

 the posterior thoracic limbs, the Zoaea passes into the Mysis- or 

 Schizopoda-like stage. These limbs, which now begin to function, 

 resemble the maxillipedes in being biramose swimming appendages 

 provided with setae ; they assist the maxillipedes in propelling the 

 body, and recall the limbs of the Schizopoda. At this stage the 

 pleopoda develop. 



In the Brachyura and Anomura, the process of development seems 

 to be simplified so far as the rudiments of the ambulatory limbs 

 are concerned ; these limbs, which are present in the late Zoaea 

 stage, never resemble the Schizopodan limbs, but from the first 

 are uniramose and pass direct into the adult form. An exopodite 

 never develops on the rudiments of these limbs. Consequently, 

 the Zoaea stage in these animals is followed by a stage in which 

 the general form of the body resembles that of the Zoaea, but, in 

 addition, the rudiments of the five pairs of ambulatory limbs are 



