MUSEUM OF COMPARATIVE ZOOLOGY. 173 



punkte, wie man bei der Betrachtung von aussen her "wahrnimmt, in 

 cler Haut von einer Gruppe kleiner Falten nmgeben ist als wenn er eine 

 R'uhre ware und hier nach aussen mundete." 2 Among recent writers, 

 Vogt und Yung ('88, p. 404) mention and figure the "cerebral canal," 

 without a more particular description of its structure or morphological 

 relations. 2 



The histological study of the canal shows some features of interest. 

 Its entire surface is lined by an extremely thin cuticula, which appears 

 under high powers merely as a double contour, pierced by numerous 

 short cilia. The cells of the ventral wall of the canal have the appear- 

 ance of ordinary hypodermal cells, except that they bear cilia. The 

 dorsal wall is made up of similar cells near the mouth of the canal, 

 but these become higher as the brain is neared, until at the middle of 

 the canal they have assumed the form of a high columnar epithelium 

 with large nuclei. This condition is preserved up to the surface of the 

 brain. When examined more closely, these cells are seen to be filled 

 with granules of a highly refractive nature, especially at their distal 

 ends, and may be regarded as the source of the more or less extensive 

 coagulum always found at the basal end of the canal. We have here, 

 then, the secretive portion of this organ. 



In cross sections of the canal (Plate III. Fig. 26) one sees clearly a group 

 of muscular fibres which is deflected from the circular layer of the body 

 wall and encircles the canal in the form of a sphincter (spht.), which, 

 although most marked at the opening of the canal, is present along its 

 entire extent. The function is evidently to prohibit the entrance of 

 extraneous matter during the forward motion of the animal, and to 



1 The Italics are not in the original. 



2 P. S. — Since writing the above, I have obtained access to a preliminary com- 

 munication by Spengel (77), and find that in this he has maintained " die Existenz 

 eines vom Gehirn zur Basis der Tentakeln fiihrenden, offenen Canales." Spengel 

 was thus the first to arrive at the true form of this structure, but I cannot find that 

 he has anywhere given a more detailed account of its morphological or physiologi- 

 cal character. In the same paper he says : " Das Gehirn stellt sich als eine knopf- 

 artige Verdickung des diesen Canal auskleidenden, mit der Epidermis zusammen- 

 hangenden Epitheles dar." Against this interpretation it may be said that the 

 embryological evidence of Hatschek ('83) makes it probable that the canal is sec- 

 ondarily formed. Furthermore, a histological examination of the parts shows that 

 the brain is less closely connected with the cerebral organ than appears super 

 ficially, since the brain capsule separates the two completely, except at the entrance 

 of the anterior commissure, which furnishes the nervous supply to the organ in 

 question. A full discussion of these relations follows the histological description 

 of the cerebral organ which is given later. 



