POLYCHAETA 285 



show at once metameric segmentation (Fig. 198), first as pairs of solid 

 blocks, then with cavities, to form the somites. Each of these box-like 

 mesodermal segments has then an inner wall which is applied to the 

 gut (splanchnic mesoderm) and an outer (somatic mesoderm) lying 

 under the ectoderm. The right and left rudiments meet in the middle 

 lane and are only separated by the dorsal and ventral mesenteries 

 which are formed by their apposed walls, while the anterior and 

 posterior borders of each segment are septa. At the same time the 

 adult nephridia develop from ectoderm rudiments and the blood 

 vessels differentiate in the septa and mesenteries. 



A 



B 



Fig. 198. Development of PoZy^orJm^. After Woltereck. A, Trochosphere 

 with rudiment of prostomium and trunk. B, Metamorphosing larva with the 

 prostomium and trunk brought close together by the contraction of the 

 longitudinal muscles and the umbrella of the trochosphere shrivelled and 

 about to be discarded. Three segments only of the trunk are shown, brn. 

 brain ; e. eye ; m.l. longitudinal musculature ; m.l. part of the same which by 

 contraction brings the prostomium and trunk rudiments into contact; M. 

 mouth ; nep. protonephridium with solenocytes ; pre. prostomium ; prt. pro- 

 totroch ; mtr. metatroch ; oe.c. oesophageal commissure ; ten. tentacle. 



The advanced larva (Fig. 198 A) thus consists of two rudiments of 

 the adult body, separated by the body of the larval trochosphere. 

 They are joined by a pair of longitudinal muscles and of nerves, and 

 in one species of Polygordius metamorphosis of the larva into the 

 adult is brought about by the shrivelling up of the larval tissues and 

 the drawing together and the union of the head and trunk assisted by 

 the contraction of these muscles (Fig. 198 B). The larval mouth re- 

 mains in the adult. After metamorphosis the animal sinks to the 

 bottom and begins its adult life. 



