180 



HOMOLOGY 



dages are provided with exopodites and endopodites ("Mysis" 

 stage, Fig. 78). 



Even in the early stages the body is covered by a carapace 

 of chitin. This is by no means as heavy and tough as in the 

 adult, nevertheless, it is highly resistant and unyielding. 

 Growth of the body thus results in an organism with a covering 

 too small for it it outgrows its clothes. The chitin carapace 

 then splits along the mid-dorsal line and the organism detaches 

 its muscles and pulls itself out of its cramped quarters. A new 



a 



FIG. 79. Stages in the early development of lobster homologous with the nauplius 

 larva of the copepods. (From Parker and Haswell after Lang.) 



chitin covering is then secreted which lasts until continued 

 growth demands a new change. This process of moulting 

 termed ecdysis is characteristic of the Crustacea and continues 

 at lengthening intervals throughout the life of the individual. 

 The " soft- shell " crab has just shed its old coat and has not yet 

 produced a new one. 



The lobster's development differs from that of more general- 

 ized Crustacea in that the embryo does not leave the egg mem- 

 brane as a nauplius larva, but continues its embryonic devel- 

 opment within the egg membrane until it has grown into the 

 form of the parent. It then leaves the egg as a young lobster 

 (Figs. 79, 80) and grows by successive moults into the adult. 



