234 



ZOOLOGY 



growth. In the earthworm (Oligochaeta) the blastopore of the 

 gastrula forms the mouth of the adult worm. In Nereis (Poly- 

 chaeta) the blastopore closes by growth, and the stomodaeum 

 and proctodaeum arise by ectodermic in vagi nations which finally 

 become continuous with the entoderm of the archenteron (Fig. 

 106, D, of Polygordius). A ciliated, free-swimming larval stage 



-S.C.. 



st. 



FIG. 106. Diagrams of stages in the metamorphosis of Polygordius, a primitive annelid. Ecto- 

 derm throughout is represented as nucleated without cell boundaries; the entoderm has the cell- 

 boundaries shown, and the mesoderm is diagonally shaded. A, gastrula; B, same with blastopore 

 closed; C and D represent formation of stomodaeum and proctodaeum from ectoderm; E, Tr echo- 

 sphere stage showing formation of segments in the posterior portion; F, adult (sagittal): G, adult 

 (transverse), a, archenteron; bp., blastopore; br, brain; c, coelom; d, dorsal; di, dissepiments; m, 

 inesenteron; pr., proctodaeum; s.c., segmentation cavity; st, stomodaeum; v.n., ventral nerve chain; 

 t, zone of formation of nerve segments. (After Fraipont.) 



Questions on the figures. Trace the behavior of ectoderm and entoderm in 

 these figures and determine what structures each seems to give rise to. What is a 

 trochosphere? Distinguish between somatic (body) and splanchnic mesoderm. 

 (See 58.) 



ensues, known as a trochosphere (Fig. 1 06, E) . The trochosphere 

 may be looked upon as representing the anterior or head end 

 of the adult. The later metamorphosis to the adult condition 



