THE OPEN SEA 101 



cradle made by the female only. Moreover, 

 the delicate cradle is made as a secretion from 

 two of the "arms," not as a secretion from 

 the "mantle," the fold of skin which manu- 

 factures the shell of all other Molluscs. 



Another pretty case is the egg-raft of the 

 mollusc called lanthina. This open-sea Gas- 

 teropod has a lightly built shell of a fine violet 

 colour, and when the time for egg-laying 

 comes a bubbly float is made in which the eggs 

 are embedded, and this is towed about by the 

 parent as it swims. 



THE OPEN SEA AS A NURSERY 



Another big fact must be included in our 

 picture of the open sea that it is the nursery 

 for the young stages of many shore-animals. 

 Delicate young stages which could not survive 

 for an hour in the rough-and-tumble condi- 

 tions of the shore are nurtured safely in the 

 spaciousness and easy-going uniformity of the 

 open sea. There is no better example than the 

 common Shore-Crab (Carcinus mcenas). The 

 developing eggs are carried about by the 

 mother under the shelter of her tail. Out of 

 the eggs come dainty pinhead-like, free-swim- 



