1 88 ELEMENTARY BIOLOGY. [CHAP. 



tion is transformed into the abdomen, for which reason 

 it is termed the abdominal papilla. The blastopore mean- 

 while becomes closed, the archenteric sac being no longer 

 in communication with the exterior; to meet this latter there 

 are instituted, immediately under cover of the labrum and 

 abdominal papilla, median ingrowths of the epidermis 

 which give rise to the lining membranes of the fore and 

 hind gut respectively, their apertures of invagination per- 

 sisting as the adult mouth and anus. 



The yolk does not enter conspicuously into any of the 

 above-mentioned outgrowths; it is enclosed within the ce- 

 phalo-thoracic region, which becomes thereby much dis- 

 tended. The gills and branchiostegites appear late; the 

 former as simple outgrowths of the body-wall and appen- 

 dages, the latter as lateral folds of the body wall. The 

 embryo which results from these developmental processes 

 passes through all the stages which are needed to bring it 

 very near to the form of the adult before it leaves the egg: 

 but, in the Lobster, the young, when hatched, are larvae 

 extremely unlike the parent, which undergo a series of me- 

 tamorphoses in order to attain their adult condition. The 

 larvae may frequently be obtained by opening the eggs of a 

 ' hen-lobster ' in ' berry.' They have when young a rounded 

 carapace, two large eyes, a jointed abdomen devoid of appen- 

 dages ; and the thoracic limbs are all provided with long 

 exopodites. During the later metamorphoses the abdominal 

 appendages appear, as that region of the body elongates 

 and increases in importance; the growth of the exopodites 

 of the thoracic appendages is at the same time arrested, 

 those of the chela and ambulatory legs vanishing entirely 

 as these appendages become specialised for locomotion. 



The ordinary growth, no less than the metamorphoses of 

 the Lobster and Crayfish, are accompanied by periodical 



