MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 135 



common observation that the Dibranchiata, with their remarkably perfect eyes, pursue their 

 quarry by the sense of sight. 1 " 



The structure of the preocular and postocular cirri .seems to be the same in all respects. The 

 annular grooves, which are so deep upon the anterior side, are frequently no more than grooves 

 in the epithelium upon the opposite side; i. e., the groove is formed by certain of the epithelial 

 cells being shorter than those of the remaining surface of the cirrus, the bases of all being at the 

 same level. In other portions of the cirri the grooves affect the subepithelial tissues of the 

 posterior side as well. The ridges upon the anterior face of the lower part of the cirrus are 

 very thin and flat, are closely pressed together, and lie in a plane perpendicular to the axis of the 

 cirrus. The ridges of the tip of the cirrus are not flat, nor of even thickness, are more separated 

 from each other, and turn upward around the cirrus like portions of the rim of a saucer. They 

 form little cups, open toward the tip of the cirrus. 



The epithelial cells of the cirri are all slender columnar cells. The cells of the bottoms of 

 the grooves are taller than those upon the outer portions of the ridges. . The former are about 

 <>i>/< in height and 4/< in width. They are ciliated, the fine cilia being about 14/< in length. The 

 latter cells are only 44>u in height. All possess oval nuclei irregularly located in the basal halves 

 of the cells. Occasionally a goblet mucus cell is found among the ciliated cells. Goblet cells are 

 frequent upon the posterior surfaces of the cirri and upon the unciliated portions of the ridges. 

 The basement membrane upon which the epithelium rests is remarkable for its sharp outline and 

 irregular surface. One can scarcely imagine a surface more wrinkled and pitted in a minute 

 way. The outer surface of the epithelium, however, does not repeat this irregularity. 



The subepithelial tissue of the ridges is very scant, what little there is consisting of fibrous 

 and elastic connective tissues, and containing few muscle fibres. 



From the structure of the cirrus, the side bearing the high ridges evidently corresponds to 

 the inner side of the cirri of the other groups, although it is turned forward. As a matter of fact, 

 if the ocular cirri were pulled forward until they were parallel to the digital cirri, the now anterior 

 sides would then be inner. For the sake of convenience in description and the comparison thus 

 introduced, I shall speak of the anatomically anterior side, when needful, as the inner side. 



The arrangement of musculature is practically that which has been described as typical of 

 the digital tentacles, except that it is even more regular in the ocular tentacles. As portions 

 of the ridges are not constructed for adhesion there is no interruption of the arrangement of the 

 muscles upon the inner side of the cirrus. Radially arranged longitudinal muscles surround the 

 nerve, although this is, as in the former case, nearer the inner side of the cirrus than the outer. 

 The two layers of circular muscle fibres and the outer longitudinal muscles pass uninterruptedly 

 around the cirrus. 



The ocular cirri are especially well supplied with rather large blood vessels. It would be 

 interesting to know if the arteries described by Willey as going to the eye do not also give off 

 branches to the ocular tentacles, or if the latter are supplied from the tentacular artery. Upon 

 the inner side of the nerve is an artery which corresponds in position to the artery of a digital 

 cirrus. Several other arteries, perhaps branches of this, lie near the nerve. A large vein lies 

 between the artery and the inner side of the cirrus. 



The structure of the nerves of the ocular tentacles is notable. (Fig. 64 and 65, N. ) The nerves 

 of the preocular and postocular tentacles arise as branches of a nerve which springs from the 

 side of the pedal ganglion, the remainder of it being distributed to several of the digital tentacles. 

 (Fig. 41, 11.) A cross section of the ocular tentacles shows that their nerves are of unusual size. 

 Near the tip of the cirrus the nerve has the same character as the nerves of the cirri already 

 described. It has an outer layer of ganglion cells, thickened in each segment into ganglia. 

 Shortly, farther from the tip of the cirrus, numerous bundles of nerve fibres join themselves to 

 the inner side of the nerve trunk. As these pass toward the base of the tentacle they seem to 

 gradually enter the nerve trunk, their places being taken by other nerve bundles from the outer 

 parts of the cirrus. Thus a kind of accessory nerve is formed which extends from near the tip 

 of the cirrus to its base along the inner side of the primary nerve trunk. (Fig. 65, N'.) These 



