coe: nemeeteans of west and northwest coasts. 51 



In Carinella and other paleonemerteans a delicate nervous plexus 

 lies between the basement membrane and outer circular muscular 

 layer. In Hubeechtia this plexus becomes very massive. 



In all cases the nervous layers consist of very delicate nerve librils 

 and a few small nerve cells supported by a delicate fi-amework of 

 connective tissue. 



Dorsal nerves. — In all nemerteans a dorsal nerve (PI. 7, figs. 55, 

 66), usually situated immediately outside the circular muscles, arises 

 from the dorsal brain commissure and extends in the median line 

 throughout the whole length of the body. 



An inner dorsal nerve, situated internal to the circular muscles 

 and above the proboscis sheath, occurs in many forms (PI. 7, tig. 55). 

 This nerve usually arises fi'om branches fi'om the outer, or principal, 

 dorsal nerve, and does not originate from the brain directly. It 

 innervates the proboscis sheath. 



Ventral nerve. — In Carinoma, Carijstomella, and certain other 

 paleonemerteans a distinct ven- 

 tral nerve is situated in the me- 

 dian line immediately external 

 to the inner circular muscles of 

 the body walls in the esopha- 

 geal region (PL 7, tigs. 55, 56 ; 

 PI. 13, tig. 82) . In the anterior 

 portion of the body this nerve 

 may lie external to the outer 

 circular muscles (PI. 13, tig. 81). 

 It apparently arises from the 

 nervous plexus above men- 

 tioned, and its development is 

 correlated with increase in size 

 of the inner muscles. 



Cephalic nerves. — The ocelli 

 and other sense organs of the 

 head are supplied with numer- 

 ous conspicuous nerves. In 

 most species these cephalic 

 nerves arise from the anterior 



faces of the dorsal ganglia, although in some forms (Text-fig. 18) 

 they are supplemented by similar branches fi'om the ventral brain 

 lobes. • 



.jrc 



Fig. \%.— ZygeupoUa littoralis. Diagram 

 of anterior portion of body, sliowing ce- 

 phalic nerves and relation of cerebral 

 sense organ (cso) to ciliated pit (rj)) on 

 side of bead; rli, rhyncliodaeum ; hr, 

 brain ; In, lateral nerve ; m, mouth. After 

 C. B. Thompson. 



