184 



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 tissues from the exo-skeleton and pulls itself out of its cramped 

 quarters. A new chitin covering is then secreted, which lasts 



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

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



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 

 general form of the parent. It then leaves the egg case (Figs. 

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



