136 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



nerve bundles are not surrounded by ganglion eells like the nerve trunk, so it is easy to distin- 

 guish the boundaries of the latter in both longitudinal and cross sections. Nevertheless, they 

 contain numerous nerve cells lying singly or in groups, some of the groups forming what one 

 might call strands of nerve cells, parallel to the nerve bundles. At the base of the cirrus all the 

 lateral bundles have entered the nerve trunk, which is here of the ordinary size. The nerve, as 

 a whole, is enlarged in each segment of the cirrus by aggregations of nerve cells, the primary 

 nerve trunk being the part most affected. From each ganglion twelve to sixteen nerves pass 

 radially to different parts of the segment, especially to the lamella-like ridge of the inner side. 

 These nerves are very large and distinct. The nerves passing to the ridges can be traced to 

 directly beneath the basement membrane of the epithelium. I could not determine any direct 

 connection of the nerves with cells of the epithelium, although, in view of the peculiar character 

 of the tentacles and the epithelium of the grooves, and the very liberal and conspicuous inner- 

 vation of these regions, it seems probable that future research will reveal special nervous elements 

 in the epithelium. 



The most curious feature of the ocular tentacles is yet to be described. The tips of the cirri, 

 consisting of several segments, or of a single terminal segment, break very readily. The ease 

 with which the segments break off is explained by the presence in the cirrus of breaking planes, 

 as I have called them. In longitudinal sections lines are seen stretching across the cirrus from 

 groove to groove, along which the connective tissues are weak or discontinuous. (Fig. 64, X.) The 

 planes correspond to the grooves between each two segments. The longitudinal muscles and the 

 nerve cord are not broken, but the muscles ;it least break very easily along the planes. Occa- 

 sionally connective tissue nuclei are gathered along the breaking planes, though this may be a 

 coincidence rather than a structural character. 



At all events, there seems to be here a provision for the amputation of segments of the preocular 

 and postocular cirri with considerable ease. It would seem as if the retraction of the cirri within 

 their sheaths at the slightest touch, as already quoted from Willey's published observations, 

 would protect them from injury. And especially so. as they are situated in a nook under the 

 auricle of the hood, back of the cephalic sheath, and above the eye, so that it seems as if they 

 need be only partly retracted within their sheaths to be completely sheltered. I feel quite sure, 

 from the constancy of these structures between all the segments of my sections, that they are not 

 artifacts, although I do not by any means deny such a possibility. If they are natural struc- 

 tures they appear to be a mechanism providing for the common and easy loss of (but not self- 

 amputation, necessarily) terminal segments of the ocular tentacles, and they also point to the 

 possibility <>f rapid regeneration of the lost portion, as is the case in other animals in which pro- 

 vision is made for the easy loss of certain parts of their bodies. But it is also very strange that 

 Nautilus should possess such a mechanism in the ocular tentacles when it also has the ability to 

 retract them quickly upon a slight stimulus. 



The innervation of the ocular tentacles, as well as their position and structure, leaves no 

 doubt but that they are members of the digital series which have become modified for sensory 

 functions. 



To summarize: 



The digital tentacles of both sexes are exactly alike in number, distribution, and structure. 

 Their fused sheaths form the cephalic sheath, a fleshy wall surrounding the anterior portion of 

 the head. Upon the inner side of the cephalic sheath of the female, above the funnel, is a 

 lamellated region for receiving the spermatophore. 



The superior labial tentacles are alike in position and structure in both sexes, but are less in 

 number in the male than in the female. 



The inferior labial lobes and tentacles are present in both sexes, but are quite different in 

 each. In the female the lobe is large and muscular. Upon its anterior edge are numerous 

 tentacles, some of which develop cirri, while others form lamellae, at the bases of which are certain 

 sensory pits. In the male the lobe and its tentacles are represented by Van der Hoeven's 

 organ. The laminae of this correspond to the cirri plus the lamellae of the inferior labial lobe 

 of the female, while the gland is not represented in the latter organ. 



