So 



Invertebrata. 



Sub-kingdom VI, Mollusca. This division in- 

 cludes all such forms as oysters, whelks, snails, and 

 cuttlefishes. Most of these are aquatic, and in none 

 is there an inner skeleton (except some small gristly 

 organs in cuttlefishes), nor are there 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 most mol- 

 luscs after the egg-stage, is as small ciliated, worm-like 

 larvae, having at their head expanded lobes (fig. 47, v) 



FIG. 47- 



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

 B, c, Larva of Molluscs, showing the ciliated velum z/, and the rudi- 

 mental foot,/, / ; c, ciliary circle ; /', intestine. 



richly clothed with cilia and resembling the trochal 

 disks of a rotifer (fig. 39, c\ or the tentacle-bearing 

 basis of the moss-polyps (fig. 47, B). This process 

 (the velum) is lost in the adult in general, but is 



