150 INVERTEBRATA CHAP. 



the edges of the head and trunk blastemas and of the prototroch. The 

 oesophageal and main retractors, and the three sub-epithelial muscles, 

 contract and pull the head blastema down until it is in contact with 

 the upper end of the trunk blastema. The prototroch is cast off as a 

 crumpled collar. 



The globular stomach is to a large extent dragged back into the 

 trunk by the peristaltic swallowing action of the visceral muscles, 

 which are not closely adherent to the gut wall. The sphincter 

 separating the end-gut from the stomach closes ; the remains of 

 the stomach are then broken up and devoured by phagocytes and 

 the renewed oesophagus comes into contact with the end-gut. Thus 

 the intestine is again completed. 



The retractors of the oesophagus and the main retractor and 

 elevator muscles are broken through by the intense contraction, 

 and absorbed, but the sub-epithelial muscles persist and become 

 continuous with the ventral and dorsal longitudinal musculature of 

 the adult. 



The first pair of protonephridia break up and disappear, and so 



FIG. 110. Polygordius neapolitanus immediately after metamorphosis. 



(After Woltereck.) 



Letters as before. In addition, Ti.b, hinder limit of head blastema ; t.h, frontal limit of trunk blastema. 



do the heads of the second pair of nephridia, but the U-shaped 

 tube of each of these persists as a nephridium of the first segment 

 or peristomium. 



The two coelomic sacs which are contained in the peristomial 

 segment, diverge in front, and between their dorsal ends a section of the 

 blastocoele projects backwards and becomes continuous with the 

 dorsal and ventral blood-vessels ; the vessels themselves indeed are 

 but the spaces between right and left sacs in successive segments. 

 The lining of these blood-vessels is provided by free wandering 

 amoebocytes which apply themselves to their walls (mes, Fig. 111). 



Immediately after the metamorphosis brain and pharynx are in 

 close contact, and the blastocoele in this neighbourhood is squeezed 

 out, but later a space between them again develops, which we 

 may term the schizocoele (schiz, Fig. 111). The pharynx be- 

 comes surrounded by a thick mantle of cells derived from the 

 descendants of 3a 2 and .">b 2 , and this mantle becomes split into an 

 inner and outer layer. From the latter are derived radiating 1 

 muscles ; these are inserted at one end in the ectoderm, while in 

 the other direction they traverse the space between inner and outer 

 layers and insert themselves in the oesophageal cells. The inner 



