68 PRIMITIVE ANIMALS 



may show many points of resemblance which are lost 

 in the adults. 



A larval form, yet more widely spread than either 

 of the Crustacean larvae mentioned, is the Trocho- 

 sphere larva (Fig. 15), characteristic of Annelids of all 

 kinds and curiously enough of some Molluscs. The 

 Trochosphere is a little spherical transparent organ- 

 ism which propels itself, generally near the surface of 

 the sea, by means of a band or circlet of cilia,- which is 

 typically situated in front of the mouth, and is known 

 as the preoral ciliated band. The sense organ is a 

 tuft of stiff hairs upon the upper pole of the organism, 

 known as the apical organ. There is a curved 

 alimentary canal, the mouth being placed laterally 

 and the anus at the lower pole. Internally the space 

 between the ectoderm and the gut is occupied by 

 small muscles, wandering cells, and often a pair of 

 excretory tubes or nephridia. In addition to these 

 structures, which make up the active organization of 

 the larva, a solid mass of cells is present on each side 

 of the hind end of the body (Fig. 15 c. mes.\ which are 

 destined to grow into paired segmented bands and to 

 form all those mesodermal structures of the adult 

 organism which are associated with the coelom, viz. 

 muscles, and reproductive organs. Thus in the transi- 

 tion which occurs from the Trochosphere larva to the 

 adolescent worm, all the larval organs except the 

 skin and the gut may be thrown away or absorbed 



