MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 115 



Beneath the epithelium just described is a thick basement membrane. This is exceedingly 

 uneven, being full of small pits. The unevennesses of the basement membrane, however, are not 

 copied by the surface of the epithelium. Over the remainder of the cirrus the basement 

 membrane is very thin. The transition from thick to thin basement membrane is as sudden and 

 marked as the transition from high to low epithelial cells, and occurs at the same place. (Fig. 72.) 



Many of the slender cells of the inner surface of the ridge taper to a tine thread-like base, 

 which often seems to penetrate the basement membrane and to be continued a short distance 

 toward the center of the cirrus as a slender fibre, but it is not possible to be sure of this because 

 of the unevennesses of the basement membrane. What appears to be a fibre penetrating the 

 membrane may be only the slender basal portion of a cell entering one of the pits of the 

 membrane. We can not help but suspect from their shape, position, and structure that these 

 cells are sensory as well as secretory, but the determination of this question will require specially 

 prepared material. Here, as in other places where the histology of Nautilus is described, 

 descriptions of the tissues are given as they have been found under certain (rather unfavorable) 

 conditions, and no attempt is made to insist upon any doubtful interpretations. 



Variations in the shape and structure of the tentacles are quite common and take place in 

 four directions, as far as my observation extends. 



1. The free ends of the sheaths may be more or less fused; fusion may occur between any 

 two sheaths, but is most usual among those nearest the hood. 



2. The free portion of the sheath is sometimes split into two halves, leaving the cirrus 

 projecting between them. This may be the result of injuries received by the animal early in 

 life, but no trace of any injury is shown by any other part and the innermost digital tentacles 

 are always the ones to be affected; from their position we should expect these tentacles to be 

 rather less exposed to injuries than any others. 



3. The opening through which the cirrus projects from the sheath may be displaced. 

 Usually the displacement is toward the inner side and the opening is found a few millimeters 

 below the tip of the sheath. But it may be at the very base of the sheath, which then projects 

 in the usual manner, but with closed tip, while the greater portion of the cirrus lies outside the 

 sheath. 



Less frequently, and only among the dorsal tentacles, the openings are upon the outer side 

 of the sheath. In one specimen one of the cirri of the hood extended from its sheath 10 milli- 

 meters back from the edge of the hood. The hood-cirrus of the other side was normal, while 

 the cirri of the two neighboring tentacles both projected from the sides of their sheaths several 

 millimeters back from their tips. 



•i. The cirrus may be entirely absent and the sheath closed. This occurred in but one 

 specimen. Where the cirrus of the left side of the hood should have projected, only a nipple- 

 like projection of the integument was seen. Upon slitting the hood back of this projection the 

 cavitj T of the sheath was found empty, without the least trace of a cirrus, and not extending the 

 usual distance into the hood. 



The more common variations of the tentacles were those of fusion of the sheaths with one 

 another. The general tendency, in fact, seems to be toward more complete fusion and increasing 

 solidity of the cephalic sheath. 



B. — Inner Tentacles of the Female. 



If now we open the cephalic sheath or cut it away we see that the mouth parts of both male 

 and female Nautili are surrounded by still other groups of tentacles. We see, further, that 

 these groups are evidently more specialized than the digital tentacles, and that the groups are 

 not alike in male and female, either in number, shape, or position, while their specialization is 

 of a higher degree in the male than in the female. It is therefore impracticable to attempt a 

 parallel description of the inner tentacles of the two sexes. The conditions are more simple in 

 the female, so I shall attempt their description first. 



The tentacles about the mouth parts are designated by the general name "labial tentacles." 

 Vol. 8— No. 5 2 



