72 LIFE IN THE SEA [PART I 



these animals impels me to this technical description). There 



are thousands of species of copepods in the sea and it is certain 



that there are hosts of new forms which have not yet been 



described. 



\ 



"^ The MoUusca belong characteristically to the benthos and only 



appear transitorily in the plankton. Many molluscs produce larvae 

 which are not pelagic, but there are others, such as the hosts of 

 cockles, mussels, periwinkles, and other shellfish, which produce 

 pelagic larvae which at times appear in the tow-nets. It is by 

 the production of these free-swimming larvae that the molluscs 

 mentioned are able to distribute themselves, just as animals which 

 have a fixed habitat on the sea bottom are generally distributed 

 by means of larvae which are carried about in the sea. A '' strike " 

 or settlement of shellfish such as the mussel and cockle occurs 

 when the fi:ee-swimming larvae are abandoning their pelagic mode 

 of life for that on the bottom. Some of the gasteropods (the 

 shellfish with spiral shells) produce eggs which in many cases are 

 enclosed in cocoons and do not float in the sea, but in this group 

 also there are many which have pelagic larvae. There are also two 

 groups of Mollusca which are planktonic animals for the whole 

 period of their lives. One of these, the Heteropods, are character- 

 istically inhabitants of warmer seas, but the other, the Pteropods, 

 are most abundant in the polar and subpolar regions, and play an 

 important part in the economy of the sea. These creatures, the 

 winged shells {Farfalle di Mare), are usually enclosed in thin shells 

 fi-om which project two fins or vanes by means of which the animals 

 are impelled by the force of the currents. It is these latter animals 

 which furnish the greater part of the food of the whales. Of the 

 other mollusca there remain two small gi^oups, the Chitons and 

 the Scaphopods, which are benthic, and the cuttlefishes and 

 squids, which are pelagic. These latter are the most highly de- 

 veloped and formidable invertebrates in the sea. Indeed it is only 

 the sperm whales which are capable of dealing with the giant 

 calamaries or squids. But they are active animals which, though 

 in some cases pelagic, belong rather to the nekton. 



Among the heterogeneous groups of Worms ("the cross of 

 suffering for systematic zoology") there are only a few groups 



