COE: NEMEETEANS OF WEST AND NORTHWEST COASTS. 



47 



Nervous System. 



Central nervous system. — In many of the smaller translucent 

 species of Amphiporus, Tetrastemma, and other hoplonemerteans 

 the form and position of the brain, sense organs, and jjrincipal nerves 

 can be studied in life, but in the vast majority of forms recourse 

 must be had to methyl blue preparations or serial sections. In many 

 species the brain and lateral nei'ves are light red or yellowish in 

 color when living. 



In the head of all nemerteans a four lobed brain constitutes the 



Fig. 16.— Carclnoyiemertes epialti. Transverse section of body immediately back 

 of brain, sliowing posterior chamber of proboscis {pc) firmly imbedded in sur- 

 rounding connective tissue ; ic, intestinal caecum ; sg, submuscular glands. 



most highly specialized nervous center, and fi'om tliis brain a pair of 

 lateral nerves extend to the posterior end of the body, where they 

 usually unite with each other above the end of the alimentary canal. 



The })Osition of the brain and lateral nerves with respect to the 

 layers of the body walls is very different in the various orders, and 

 forms one of the most important taxonomic features. In Pkocaki- 

 NiNA and CarinijSta the brain lies completely in the integument, 

 while in Carinella and most other paleonemerteans it lies imme- 

 diately beneath the basement layer of the integument. 



In the Hoplonemertea the brain lies imbedded in the body paren- 



