XVI 



LIFE-HISTORIES 



335 



on tadpoles, that a thyroid diet stimulates differentia- 

 tion and hinders growth, while a thymus diet inhibits 

 differentiation and lets growth go on. 



The general idea is that the span of life is like a dis- 



FIG. 111. DIAGRAMMATIC REPRESENTATION OF THE LIFE-HISTORY OF 

 THE COMMON SHORE-CRAB (Carcinus mamas). 



1. The spherical eggs with chitinous envelopes. They show partial 

 peripheral segmentation around a central core of yolk. They develop 

 attached to the swimmerets of the mother. 



2. The Zpaea larva, which is hatched from the egg and lives a free- 

 swimming life in open water. It is to begin with about the size of a pin's 

 head. It has paired compound eyes, a spine on the dorsal surface of 

 its cephalothoracic shield (see the figure of the crayfish, p. 250). It has eight 

 pairs of appendages from the antennules to the third maxillipedes. 

 The segmented abdomen sticks out behind. It feeds and grows and moults 

 and eventually becomes a Megalopa (3). 



3. The Megalopa is more like a lobster than a crab, for the abdomen is 

 still in a line with the cephalothorax. The forceps and walking-legs are 

 developed. 



4. The abdomen is subsequently tucked upwards and forwards under 

 the cephalothorax ; the adult form has been assumed ; the young crab 

 has ceased to be much of a swimmer and has become a shore-animal. 



