ANNELIDA 



235 



In Nereis (Polychaeta) the blastopore closes by growth, and the 

 stomodaeum and proctodaeum arise by ectodermic invaginations 

 which finally become continuous with the entoderm of the arch- 

 en teron (Fig. 106, D, of Polygordius). A ciliated, free-swim- 



FiG. 106. Diagrams of stages in the metamorpliosis 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. At gastrula; B, same with blastopore 

 closed; C and D represent formation of stomodsum and proctod'aeum from ectoderm; E, Trocho- 

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

 (transverse). «, archenteron; 6^., blastopore; 6;*, brain; c, coelom; J, dorsal; di, dissepiments; m, 

 mesenteron; pr,, proctodaeum; s.c, segmentation cavity; st, stomodaeum; ».w., 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 

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

 (See §58.) 



ming larval stage ensues, — known as a, trochophore (Fig. io6, £). 

 The trochosphere may be looked upon as representing the ante- 

 rior or head end of the adult. The later metamorphosis to the 

 adult condition involves the reduction in size of the enormous 

 anterior region, and the growth of segments at the posterior end, 



