454 



ANNELIDA. 



the animal fixes itself to the stones on which it creeps. The circular muscles 

 of the body-wall are less developed than the longitudinal. There are also 

 obliquely -placed muscular sheets passing from the ventral body-Avail to the 

 lateral, and dividing the body-cavity into two lateral and one median chamber. 

 The insertion of these oblique muscles is associated with the division of the 

 longitudinal muscle into four bands one dorsal, two lateral, and one ventral. 



Prw. 



Ms 



FIG. 361. Development of Polygordius (after Hatschek). a, young trochosphere larva ; ,s> 

 apical plate with pigment spots ; Prw preoral, Pow postoral ring of cilia ; mouth ; A anus ; 

 Ms mesoderm ; Kn larval kidney (pronephros or head-kidney), b, older larva with commenc- 

 ing segmentation of the body, c, older stage ; the body is elongated and vermiform, and 

 segmented; H Wk posterior circle of cilia; Af eye spot; F tentacle. 



The ventral nerve cords are widely separate, and are placed, as is also the 

 cerebral ganglion, in contact with the ectoderm immediately beneath the ventral 

 insertions of these oblique septa. They are without ganglionic swellings. 

 There are two contractile vesicles at the base of the tentacles, which drive 

 a colourless fluid into the hollow tentacles and so distend them. 



The oesophagus extends for thirteen or fourteen segments, and is followed 



