Molluscs. 



79 



any limbs, properly so called, in the whole group 

 The outer tunic of the body is generally thick and 

 extended to form a leathery envelope or mantle, the 

 outer surface of which secretes a shell of carbonate of 

 lime for the protection of the animal. 



The earliest condition of existence of a mollusc, 

 after it has left the egg-stage, is as a small ciliated, 

 worm-like body having at its head an expanded lobe, 

 richly clothed with cilia and resembling the trochal 



Larval forms of Worms and M olluscs. A, Larva of a Gephyrean Worm ; 

 B, c, Larvae of Molluscs, showing the ciliated velum v, and the rudi- 

 mental foot, f. 



discs of a rotifer, or the tentacle-bearing basis of the 

 moss-polyps (fig. 47, B). This process is lost in the adult 

 in general, but is interesting as one of the many eviden- 

 ces of the relationship between worms and molluscs. 

 The shells secreted by molluscs consist of one, 

 two, or several valves, or pieces, and are very various 



