COE: NEMERTEANS OF WEST AND NORTHWEST COASTS. 



61 



this order the dorsal ganglia are bilobed posteriorly, the ventral 

 lobe with its large fibrous core continuing directly into the sense 

 organ. This projects freely into the large lateral blood lacuna, with 

 which it is com2:»letely surrounded except on its anterior face. • 



Lateral sense organs. — These occur in a well developed condition 

 only in the Paleonemertea, although similar organs, l)ut much less 

 highly specialized, are found in Micrella (Punnett, :01'») and per- 

 haps also in Zygeupolia 

 (Thompson, :02, p. 686). 

 When most highly spe- 

 cialized (Text-fig. 26) 

 they ai-e oval or circular 

 areas of modified sensory 

 epithelium provided with 

 a special musculature and 

 capable of considerable 

 movement. The^^ can be 

 raised as elevations above 

 the surrounding integu- 

 ment or depressed as pits. 

 A single pair of these 

 sense organs lie on the 

 lateral margins of the 

 body in the immediate 

 vicinity of the nephridio- 

 pores. In Cakinella 

 (Text-fig. 26; PI. 15, 

 fig. 91), Caeinomella 

 (PI. 7, fig. 56) and most 

 other paleonemerteans, 

 the sense organs are very 



highly develoi)ed. They are provided with branches from the 

 adjacent lateral nerves, are free fi-om pigment, and usually have 

 numerous gland cells. The sensory cells are very slender, closely 

 packed, and provided with long cilia. For details of structure see 

 the specific description of Carinornella lactea. 



Frontal sense organs. — At the tip of the snout and immediately 

 dorsal to the proboscis pore occur, in many of the ITeteronemertea, 

 one to three circular pits lined with epithelium of a highly sensory 

 nature. In Cerebratulus and Miceura there are often three such 





Fig. 26. — Carinellafrenata. Portion of transverse 

 section of body througli lateral sense organ, show- 

 ing single layer of speeializeil sensory cells lining 

 sensory pit ; In, lateral nerve ; bm, basement 

 membrane. 



