VII 



ANNELIDA 



149 



these intermediate cells, however, just at the margin of the head 

 blastema, become transformed into gland cells (gl, Fig. 109), which 

 are almost certainly of an excretory nature. 



A similar contact takes place between the lower end of the 

 prototrochal ring and the front end of the new ectoderm, forming 

 the trunk blastema (arising chiefly from the descendants of the cell 

 2d 222 ); so that the cells forming the hyposphere (descendants of 

 3 c 2 P ap anc { 3 c j2pap) j-, ave & \ so b een absorbed. 



The larval oesophagus gradually loses its cellular walls, which are 



musc.L 



x to show 



FIG. 109. Two sections of the anterior part of ,1 larva of Polygonl ! H s 



the changes supervening on metamorphosis. (After Woltereck.) 



A, frontal section just before, metamorphosis. B, sagittal section just after mrtamnri>li'>sis. 

 Letters as before. In addition, c.f, ciliated filter ; gl, excretory gland cells. The coclomic cavities c.r 

 the somites are dotted but the blastocoele has a pale uniform tint. 



replaced by new cells budded from two lateral pockets. These 

 pockets may be compared to the "imaginal discs" of insects (see p. 

 267). The renewed oesophagus is delimited from the stomach by a 

 ring of ciliated cells. From the walls of the stomach, cells are given 

 off into its cavity and degenerate, and in this way the globular 

 stomach slowly decreases in size. The boundary separating .sinm.-ieh 

 from intestine is, of course, marked by a projecting valve. This is 

 armed with stiff cilia so as to constitute a ciliated filter (c.f, Fig. 1 09). 

 When the growth of the post-oral region has reached its full 

 extent cataclysmal changes take place. The circular muscles under- 

 lying the prototroch contract so vigorously as to cut the skin uniting 



