GENERAL CONSIDERATION. 249 



In the complete series of developmental stages of the Decapoda, 

 which however is only retained in full in a very few cases, the 

 following stages can usually be distinguished passing one into the 

 other by a series of moults. 



1. The Nauplius stage (Fig. 122 A, p. 267). This stage shows 

 great agreement with the Nauplius of the Entomostraca in form 

 and in the possession of the three typical pairs of limbs, the anterior 

 limb (first antenna) being uniramose, but the two posterior limbs 

 (second antenna and mandible) biramose. A free Nauplius stage is 

 found in Penaeus and in Euphausia, among the Schizopoda. 



2. The Metanauplius stage (Fig 117, p. 254, and Fig. 118, p. 258), 

 in the form of the body, closely resembles the last stage, but shows, 

 behind the Nauplius limbs which have shifted somewhat forward, the 

 rudiments of four more pairs (in Euphausia only three). An 

 integumental fold, which arises laterally and posteriorly, is the 

 first rudiment of the dorsal shield. The posterior end of the body 

 is marked by two short prominences bearing setae (furcal processes). 

 The Metanauplius stage is the starting-point in the metamorphosis of 

 Lucifer. 



3. The Protozoaea stage (Fig. 119 A, D, p. 260, and Fig. 121 A, 

 p. 264). The pairs of limbs which appeared as rudiments in the 

 Metanauplius (first and second maxillae and second maxillipede) have 

 now developed fully. The anterior region of the body is covered by 

 the dorsal shield ; posteriorly, the body passes into a narrower region 

 combining the rudiments of the thorax and the abdomen, and already 

 showing segmentation anteriorly, while the posterior (abdominal) 

 region is not yet fully segmented. The antennae still possess the 

 Xauplius character and function as oars, as also do the biramose 

 maxillipedes. The mandibles have greatly altered ; the basal joint 

 is retained as a masticatory blade, while the distal part (palp) has 

 disappeared. The Protozoaea stage occurs in the metamorphoses of 

 the Penaeidae and Sergestidae. It is distinguished by the presence 

 of distinct furcal processes. In individual cases (Sergestes) the third 

 pair of maxillipedes may also be developed at this stage. 



4. The Zoaea stage (Fig. 119 E, p. 260 ; Fig. 123 (7, p. 269 ; Fig. 

 124, p. 272, and F'ig. 136, p. 291). This stage agrees in all important 

 characters with the preceding stage, from which it is distinguished 

 by the distinct segmentation of the posterior or abdominal region. 

 The sixth abdominal segment, it is true, often remains united with 

 the telson for some time. The limbs of the Zoaea stage are the same 

 as those of the preceding stage. In the more primitive Decapoda, 



