MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 117 



The cirri of the labial tentacles are, of course, shorter and more slender, but their structure 

 and shape are the same as of the digital tentacles. 



The structure of the inferior labial lobe is more complicated. I have retained Owen's 

 designations of superior and inferior instead of the ones suggested by Van der IIoeven, external 

 and internal, for these lobes because the former names indicate better their actual positions. I 

 have already spoken of two inferior groups of labial tentacles. Two groups there are, but they 

 are borne upon a single Lobe, the inferior labial lobe. 



The inferior labial lobe (Fig. 5, L; fig. 26) arises immediately below the buccal cone as a 

 process about 2£ centimeters broad and 8 millimeters thick. About 2£ centimeters from its base 

 the process is split into two symmetrical portions. Each portion widens out like a fan and bears 

 a single row of tentacles along its edge. (Text- fig. 2, ILL.) Each of these parts wraps itself 

 about the buccal cone internal to the superior labial lobes. The inner surface of each is concave, 

 the outer surface convex. Sheaths and cirri of the inferior labial groups are like those already 

 described for the superior labial tentacles, except that the sheaths do not project freely. The 

 middle tentacles of each inferior group are the longest; those at the outer end of the row are 

 only slightly shorter, but the inner tentacles decrease rapidly and greatty in size. A progressive 

 reduction in the development of the sheaths is also noticeable among the innermost tentacles. 

 The innermost tentacle is frequently a mere papilla 2 or 3 millimeters in height. 



The number of tentacles in each inferior labial group usually varies between ten and fourteen. 

 The number upon the opposite divisions of the same lobe frequently differs by one or two. Van 

 der Hoeven reports fourteen upon the left and sixteen upon the right side. 



At the junction of the outer portions of the lobe, upon its inner side, is an oval or polygonal 

 organ composed of sixteen to twenty closely folded thin triangular lamellae, the widest portion 

 or base of the organ being directed posteriorly. (Fig. 5, L; Fig. 20; Text-fig. 2, ILL.) Usually 

 sixteen lamellae compose the organ, which Owen supposed to be the olfactory organ of Nautilus. 



The lamella; are symmetrically disposed with respect to the median line of the body. Occa- 

 sionally there is a difference of one in the number of lamellae of the two halves of the organ. 

 The organ represented in Fig. 20 was more expanded than most, so shows the relative arrangement 

 and form of the lamella? better than is usually the case. This organ measured 12 millimeters in 

 breadth and 9 in length. The triangular lamella? are attached to the labial lobe by one edge, 

 the point opposite then projecting upward and inward. The line of attachment of each of the 

 median lamella? is parallel to the axis of the body, while the outer lamella? are attached at acute 

 angles to the axis. The outer lamella of each side usually lies somewhat above the others, over- 

 lapping them and lying in the continuation of the rows of tentacles upon the outer parts of the 

 labial lobe. A ridge across the open back of the organ connects the two outer lamellae. 



Each lamella is distinctly and closely grooved upon its outer side, the grooves reminding 

 one of those upon the cirri. Less numerous, distinct, and regular grooves are seen upon the 

 inner sides of the lamella?. A large nerve runs through the center of each lamella to its tip. 

 The nerve possesses an outer layer of ganglion cells which are evenly distributed upon its surface 

 and not grouped in ganglionic masses as they are upon the nerve of a cirrus. 



Both surfaces of the lamella? are deeply pitted, and all portions are covered with a highly 

 glandular epithelium. 



The musculature of a lamella has essentially the same arrangement as that of a cirrus. In 

 consequence of the flattened form of the lamella? their longitudinal muscles extend outward to 

 the edges upon either side of the nerve. The transverse fibers, which are radially arranged 

 about the nerve in the cirri, here, therefore, pass directly across the thin lamella? between the 

 bundles of longitudinal fibres. External to the latter are circular muscle fibres. 



In the farthest depths of the fissure between each two lamella^ is a small opening leading into 

 a sunken organ which I do not hesitate to call sensory. These organs ordinarily consist of two 

 parts: 1. a tubular neck opening outwardly as has just been mentioned, and opening inwardly 

 into 2, a (comparatively) large cavity which extends in the solid tissue of the labial lobe at right 

 angles to the surfaces of the lamellae. (Text-tig. 3, G.) 



The whole structure reminds one strongly of a diminutive tunnel driven in both directions 



