ALASKA NEMERTEANS 19 



Cephalothrix Oersted., 

 Entwurf der Plattwurmer, Kopenhagen, 1844. 



This genus includes very long, slender, filiform species which show 

 a tendency to coil in a close spiral. Brain situated well behind tip of 

 snout ; mouth several times as far posteriorly. Head sharply pointed 

 in extension ; proboscis-pore on ventral side a little back of its extrem- 

 ity. Inner circular muscular layer very much reduced, or (commonly) 

 entirely absent, the body musculature consisting of a thin outer circu- 

 lar muscular layer and a strong inner longitudinal layer ; lateral nerves 

 situated in the longitudinal muscular layer ; cerebral sense organs and 

 cephalic furrows wanting. These worms resemble in external appear- 

 ance some of the slender Nematodes. 



4. CEPHALOTHRIX LINEARIS (Rathke) Oersted. 



Planaria linearis RATHKE, Skrivter af Naturhist. Selsk. Kjobenhavn, v, p. 



84, 1799. 

 Cephalothrix linearis OERSTED, Entwurf der Plattwurmer, p. 82, Kopenhagen, 



1844. 



A very slender thread-like species capable of great extension and 

 contraction. Individuals may be extended till they resemble a very 

 fine thread, but when disturbed commonly coil themselves into a closely 

 wound spiral. Body commonly rather thicker in the middle and taper- 

 ing toward both extremities. Head very long, acutely pointed when 

 extended. Proboscis-pore situated ventrally, some little distance back 

 from tip of snout. Mouth very far back ; commonly distant from tip 

 of snout 10 to 12 times the diameter of body; or it is perhaps 4 to 5 

 times as far back as is the brain. 



The length is subject to the greatest variation. Alaska specimens 

 commonly extended themselves 3 to 6 inches or more, but could con- 

 tract to a small fraction of this length. 



Ocelli. Wanting in adults although the embryos are provided with 

 a single pair soon after leaving the egg. 



Color. Usually pale yellow throughout, but some specimens had a 

 distinct reddish tinge, and some were gray, greenish, or pale green. 

 A median paler line, due to the proboscis sheath, appears on the dorsal 

 surface in the esophagal region. 



Habitat. The species is very common well up toward high- 

 water mark under stones in muddy places, among decaying mussels, 

 etc. The worms were commonly found where the mud was black, 

 slimy, and very foul. Scores of specimens were sometimes found 

 under a single stone. They were often associated with slender red- 



