COE : NEMEETEANS OF WEST AND NORTHWEST COASTS. 



59 



cal organs surrounded by connective tissue, and are situated beneath 

 the integument. In the center of the sense organ (Text-fig. 24) lies 

 a ciliated canal lined wdth sen- 

 sory epithelium and gland cells. 

 This canal passes to the surface 

 of the integixment, and opens 

 to the exterior of the l>ody, 

 sometimes at the summit of a 

 small papilla situated at the 

 bottom of a conspicuous pit on 

 the side of the head, as in C. 

 frenata (Text-lig. 25). The 

 central cavity of the sense 

 organ is thus fi'eely connected 

 with the water outside the 

 body, so that the cells of the 

 sensory epithehum of the canal 

 can respond to stimuli con- 

 ducted to them through the 

 water. The dehnite function 

 of the sense organs, however, 

 still remains an unsolved jirob- 

 lem. In (\ frenata the ciliated 

 canal penetrates the basement 

 membrane and extends inward 

 to the dorsal side of the brain 

 (Text-fig. 25). Several .large 

 nerves pass fi-om the dorsal 

 ganghon directly into the substance of the sense organ (Text-figs. 

 24,25). 



In its more highly specialized condition, as in the Heteronemertea, 

 the canal of the sense organ presents two more or less distinct seg- 

 ments marked off by the entrance of the ducts fi'om large clusters 

 of gland cells. The external section of the canal extends fi-om the 

 exterior of the body to the anterior glandular cells of the sense organ, 

 and is characterized by slender cells vnth. very long cilia. The 

 internal, or posterior, section extends posteriorly from the anterior 

 glandular masses and is peculiar in having a very highly specialized 

 epithelium on its lateral face, while its internal face is lined with 

 slender, ciUated cells much like those of the anterior section. 



Fig. 25. — CarineUa frenata. Portion of 

 transverse section of head, sliowing 

 deep, ciliated pit {cp) from which a cili- 

 ated sensory canal leads inward to brain 

 region, where it becomes surrounded 

 with sensory nerves from dorsal side of 

 brain to form a highly specialized sense 

 organ. 



