22 



whole of these or such part of them as are to be developed at this 

 period are completed the larva is a zoea. A physiological pre- 

 cocious appearance and growth of the uropods may take place 

 in the later protozoea stages. When the pleopods in turn have 

 become fully developed the larva is a megalopa. Although the 

 limbs at this stage are complete, several ecdyses are necessary 

 before the limbs attain the adult character. 



The pelagic Euphausiacea (fig. 5), like the pelagic Penaeidea, 

 are hatched at the nauplius stage, but they attain the megalopa 

 condition through stages which are far from being parallel. The 

 metanauplius stages lead to the calyptopis, which possesses 

 six pairs of limbs. A further series of ecdyses brings about the 

 addition of the seventh limb, and also the uropods. This stage 

 has been styled the furcilia. It is a typical protozoea. One or 

 two more ecdyses convert this into the cyrtopia, which has eight 

 pairs of appendages, and two other ecdyses bring about the 

 complete development of the limbs, and the megalopa, which in 

 this case is not very di£ferent from the adult, is established. The 

 cal}^topis is peculiar, but it is follow^ed by stages which are 

 typical j)rotozoea, and a zoea phase may be said to be absent. 



The Mysids, Amphipods, Isopods and their allies undergo 

 development in brood chambers, and are usually liberated in the 

 megalopa condition, but in some cases as zoeae. The eggs hatch 

 in some in the pouch as nauplii, in others at a more advanced 

 stage, but even in these latter the nauplius stage is indicated. 



Nebalia is reared under the protection of the bivalve carapace. 

 A nauplius stage is passed through in the egg which is hatched 

 as a zoea with all the thoracic limbs completed, and the larva 

 is liberated in the megalopa state. 



With the exception of Cyclestheria of the Phyllopoda and the 

 Cladocera, which have parthenogenetically produced young in 

 the summer, the Entomostraca are liberated as nauplii. The 

 ecdyses bring about increase in size and a gradual appearance of 

 the limbs in the usual order. 



The sum of my plea here is that I consider the stages in 

 Crustacean development can be best defined by the number of 

 appendages, which in the successive stages have reached a func- 

 tional condition. I venture to propose therefore the following 

 stages : — 



