XII 



BACKBONELESS ANIMALS 



263 



Many molluscs pass through 

 two larval stages before they 

 acquire their characteristic 

 adult appearance. The first 

 is interesting because it is 

 virtually the same as the 

 young stage of many marine 

 Annelids. It is a minute 

 barrel-shaped or pear - like 

 creature with a ring of loco- 

 motor cilia in front of the 

 mouth, and is known as a 

 Trochosphere. 



After a while this changes 

 into a more characteristic 

 form called the Veliger. It 



FIG. 81. A DEEP-SEA CUTTLEFISH 

 (After Chun.) 



One of the remarkable adaptations 

 is the development of so-called " tele- 

 scope " eyes, probably suited for 

 making the most of the dim light. 



bears on its head a ciliated 

 cushion or velum often pro- 

 duced into lobes ; the body 

 has a ventral " foot ' and a 

 dorsal "shell-gland." In 

 aquatic Gasteropods the vis- 

 ceral hump begins to appear 

 at this stage. 



The eggs of cuttlefish differ 

 from those of other molluscs 

 in their rich supply of yolk, 

 which serves for a prolonged 

 period as capital for the young, 

 and the two larval stages 

 noticed above are skipped 

 over. In land snails and slugs, 

 what comes out of the egg is 



