42 THE STORY OF ANIMAL LIFE 



adult, and adapted for a- different mode of life. 

 It is, in fact, a very common arrangement ; but 

 often these larvae are very minute, perhaps abso- 

 lutely microscopic, therefore only known to the 

 scientific observer. The two familiar instances 

 we have named are fortunately big enough to be 

 known to everyone. Now it is an axiom with 

 modern zoologists (as has been 

 explained above), that the his- 

 tory of the individual is a sum- 

 mary of the history of its ances- 

 tors ; larval forms are therefore 

 of special interest in this con- 

 nection. A very wide-spread 

 form of larva, more advanced 

 Fl k l%St* in its structure than the little 

 a typical Trocho- Gastrula that has been already 

 sphere, or ciliated named, has received the name 



larva, considerably c ,,-, , , -uru it n 



magnified, j/isthe of I rochosphere or Wheel-ball 

 mouth; the stomach (Fig. 7), because it swims round 

 ^^ g t n ro a u r g e h S ^ d found, by means of cilia, 

 transparent body. usually distributed in bands. 

 Its inner or stomach - layer, 

 forms a definite alimentary canal, and is separated 

 by a very simple mesoderm from the outside cili- 

 ated layer, which presents certain differences in 

 form, according as the creature belongs to one 

 group of animals or to another. The main char- 

 acters of the Trochosphere are, however, the same 

 in very widely differing groups. These little larvae 

 give rise to one of the most eagerly debated prob- 

 lems of zoology. Are we to suppose that animals 

 which possess a Trochosphere larva are all de- 

 scended from one common ancestor ? Or are we 

 to think that the Trochosphere is a form of body 

 very convenient for the necessities of juvenile 



