COE: NEMERTEANS OF WEST AND NORTHWEST COASTS. 57 



backward toward the median line on the dorsal surface (PI. 18, 

 figs. 111-113), but are commonly more widely separated beneath. 

 When most highly specialized, as in Drepanophorus and Amphi- 

 poriis himaculatus (PI. 18, figs. 116, 117), the grooves are conspic- 

 uous because of the Huted appearance caused by numerous trans- 

 verse ridges. They are largely devoid of gland cells and pigment, 

 and are lined with slender sensory cells with long cilia. 



Among the Heteronemertea Taeniosoma often has somewhat 

 similar oblique, Init less highly specialized grooves, while at least 

 one species ( 2\ hemprichi) of the genus has an indication of the 

 highly developed horizontal furrows of most of the Lineidae. In 

 these grooves the canals leading to the cerebral sense organs 

 terminate. 



In the Lineidae the horizontal furrows are best developed, 

 although certain members of the family have them merely indi- 

 cated or completely wanting. When well developed they are situ- 

 ated on the exact lateral borders of the head, and commonly extend 

 from the tip of the snout back to the brain or as far as the anterior 

 border of the mouth. The cerebral sense organs always connect by 

 means of a ciliated canal with the posterior ends of these furrows, 

 which are shallow anteriorly and deepest at their posterior ends. 

 In Cere BRA TULUS they are usually much deeper than in Line us, 

 and in some species cut into the tissues of the head as far as the 

 lateral borders of the brain. In C. coloratiis Biirger, however, they 

 are represented merely by slight grooves, and in other Lineidae, as 

 Zygeupolia (Text-tig. 1 8), they are wanting completely, the canals 

 fi'om the cerebral sense organs opening into slight pits on the lateral 

 borders of the head. 



Among the Pacific coast forms Micrura alaskensis and Cere- 

 bratul'us nehulostis are peculiar in having very shallow cephalic 

 furrows, while in most other Lineidae from that region they cut 

 inward well toward the brain. In most species their length is con- 

 siderably greater than the transverse diameter of the body, but in 

 C latus (PI. 4, Hg. 41), which is a remarkably short and broad 

 form, they are not more than one seventh as long as the greatest 

 diameter of the body. As a rule the furrows are deepest in those 

 forms in which they are longest, but there are certain exceptions. 

 Where the furrows are very long the canals from the cerebral sense 

 organs open somewhat in front of their posterior ends. 



