20 



the Malacostraca are arranged in vertical columns, and their names 

 as applied to the Decapoda are also indicated. The horizontal 

 lines opposite the names of the groups indicate the stage at which 

 hatching takes place, and the successive ecdyses by the step-like 

 breaks in the line. All the appendages to the left of the steps 

 in each case have reached a functional condition, or, in other 

 words, the steps refer to the ecdj^ses during which the appendages 

 to the right are completmg their development. 



It is usually assumed that the development of the Penaeidea 

 exhibits best the typical condition, and it will be convenient to 

 describe it now as serving to define the stages. The eggs of the 

 pelagic Penaeus are loosely attached to the posterior pereiopods, 

 and are hatched in the course of a day or so. The larva is a 

 simple nauplius bearing three paks of unsegmented limbs. An 

 ecdysis converts this into a metanauplius, in which the carapace 

 is indicated and rudiments of the succeeding four pairs of limbs. 

 These latter attain full develo23ment at the next ecdysis, and 

 the larva now becomes a protozoea. The next four ecdyses and 

 instars complete the segmentation of the thorax and abdomen, 

 and mark the appearance and growth of limbs 8-12, as well as 

 the uropods which appear precociously early in Penaeus as in 

 several other Malacostraca. The posterior pairs of thoracic 

 limbs have not much room for development, and m this group 

 the last or the two last may not appear, or in the case of the 

 penultimate pair ma}^ have only a temporary existence. This 

 is the stage which ma}^ be called the zoea. During the next three 

 ecdj'ses the pleopoda are completed, and the larva attains its 

 adult quota of limbs. This may be called the megalopa, and 

 giv^es place with fu?^ther ecdyses to the adult form when the limbs 

 do not undergo any further marked modification. 



These stages will be sufficientl}^ plain from the chart. It is 

 still convenient in the case of the Sergestidae to define the succes- 

 sive stages of the elaborate^ spinulated larvae by such names 

 as elaphocaris, acanthosoma and mastigopus, but it is not necessary 

 to consider them here. 



The Brachyura (fig. 2) are liberated in the condition which 

 in the Penaeidea has been styled the protozcea, that is to say, 

 the limbs are present up to and including the second maxilliped, 

 and the larva is called by the name protozoea b,y authors. At 



