294 



ZOOLOGY 



SECT. 



the muscular layers. The median cord is always, except in the 

 Heteronemertines, superficially placed. 



A remarkable apparatus connected with the nervous system 

 is that composed of a pair of peculiar structures known as the 

 cerebral organs. When most highly developed these consist 

 of a pair of ciliated tubes (Fig. 230, cil. gr.\ opening externally in 



FIG. 23<j. A, Piliclium with advanced Nernertine worm ; B, ripe embryo of Nemertes from 

 interior of pilidium. .an. amnion, or investment of the embryo ; i. intestine ; Ip. lateral pit ; 

 n. nervous system ; o>. gullet ; pr. proboscis ; st. stomach. (From Balfour, after Biitschli.) 



the region of the brain or of a pair of lobes separate from the 

 latter. This apparatus may have a respiratory function, more 

 especially for the oxygenation of the substance of the brain, but 

 perhaps it has also a sensory function. It has some resemblance 

 to the ciliated pits developed in certain Turbellaria. 



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

 more highly organised species occur in considerable numbers. 

 Sometimes they are of extremely simple structure ; in other cases 



