ZOOLOGY SECT. 



usually in a groove or slit, and ending internally in the interior of 

 a pair of posterior brain lobes, which are sometimes distinct from, 

 sometimes united with, the rest of the brain. The posterior brain 

 lobes are developed in the larva as outgrowths from the oeso- 

 phagus ; the ciliated tubes appear quite independently of them 

 as ingrowths from the epidermis of the head, the two structures 



FIG. 221. A, Pilidium with advanced Xcmerthie worm ; B, Rips embryo of Nemertes from 

 interior of Pilidium. an. amnion, or investment of the embryo ; i. intestine ; lp. lateral pit ; 

 /'.nervous system ; . gullet ; p>: proboscis ; st. stomach. (From Balfour, after Butschli.) 



only subsequently coming into relation with one another, and the 

 outgrowths from the oesophagus usually altering their cellular 

 structure so as to become converted into nerve-tissue. This 

 apparatus is probably a special arrangement for oxygenating the 

 brain substance ; but perhaps it has also a sensory function. 



Eyes are present in the majority of Nemerteans, and, in the 

 most highly organised species occur in considerable numbers. 



