THE crayfish: 



147 



winter there. In the spring they migrate back to the shore — 

 the female tarrying behind the male until the eggs of last 

 summer, which she still carries attached to her swimmerets, 

 shall be further grown. In June the young hatch out, molt, 





^A._^v.V2 





', vrii . 



■r:^'^. 



^•iU 



:^** , 



■ V 





'f. 



''i^" ft^f *^ 





Fig. 146. — Limulus polyphemus, the king-crab or horaeshoe-crab. 



and swim to the surface. The female now molts, but does 

 not spawn again for a whole j^ear ; that is, she spawns in 

 alternate years. The number of eggs carried varies with the 

 age of the female ; middle-aged lobsters may carry up to one 

 hundred thousand eggs, but the old or young ones as few as 

 three thousand. The egg as freshly laid is about 1..5 milli- 

 metres in diameter and is stored with food material, called 

 yolk, much as in the case of the hen's egg. As in the chick, 

 the development takes place, as it were, on top of the egg 

 (Figs. 147, 148). Ej^es and mouth appendages early make 



