152 



INVERTEBEATA 



CHAP. 



forth as the segmented trunk of the adult worm. Previous to this 

 period it is held in its compressed and crumpled condition, along its 

 surface to its posterior end, by extensions of the main blastocoelic 

 retractor and elevator muscles. When these break through during 

 the intense contraction which accompanies metamorphosis, the 

 straightening process begins. 



A further difference between the two species of Polygordius is 

 found in the condition of the nephridia. In P. lacteus the archi- 

 nephridia remain small and unbranched, and the only change which 

 they undergo in becoming converted into the first pair of proto- 

 nephridia is to become covered with numerous cuticular tubes — each 



Fig. 112. — Late larva of Polygordius ladeus in optical frontal section. (After Woltereck.) 

 Letters as before. In addition, ain.f, amniotic fold ; dv, larval elevator muscle ; t.fr, telotroch. 



containing a flagellum — so as to resemble, in fact, one of the soleno- 

 cytes of the first pair of protonephridia of Polygordius appendiculatus. 

 The second pair of protonephridia take the form of long tubes studded 

 all over with solenocytes, which are directed dorso-ventrally, parallel 

 with the prototroch, and which open dqrsally at the beginning of the 

 post-trochal region. 



It is obvious that the development of P. lacteus as compared with 

 that of P. appendiculatus is shortened and modified. In some points 

 indeed it reminds us of the development and metamorphosis of the 

 Pilidium larva of the Nemertinea. Thus we could compare the four 

 amniotic invaginations of the Pilidium larva to the four pockets 

 which arise from the circular groove in the Trochophore, since in 

 both cases the adult body is formed from their inner walls ; but the 



