MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 109 



lateral corners lying against the umbilici of the shell arc sometimes spoken of as the auricles of 

 the hood. (Fig. -. Ho. A.) In most specimens a slight groove runs along the middle of the 

 dorsal fascia «>t' the hood, although it is frequently absent. 



Scattered more or less evenly over the surface of the hood are numerous small papillae. The 

 possibility of these possessing a tactile function is immediately suggested, but my material shows 

 no structure to confirm it. The papillae are differently distributed upon different Nautili, in 

 some being most thickly placed upon the median portion of the hood, in others upon the lateral 

 parts. But comparing one specimen with another, the papillae may be said to be evenly distrib- 

 uted over the entire surface. 



In the anterior edge of the hood are two small openings leading into deep cavities, in each 

 of which is a cirrus exactly similar to the cirri of the neighboring tentacles. (Fig. 2, Ho.C.) The 

 presence of the cirri, the innervation and the anatomical relations of the hood, and the fact that 

 the neighboring tentacles are sometimes closely fused with it leave no doubt that the hood is 

 composed of two tentacles, the sheaths of which have become much enlarged and closely fused. 

 It seems probable that the hood of the Nautilus, aside from its other uses, serves to protect, the 

 animal when withdrawn into its shell in much the same way as the operculum of a Gastropod 

 protects its owner. It is noticeable how closely the hood of a strongly contracted Nautilus tits 

 the opening of the shell. 



A cleft of varying depth existed in the middle of the anterior edge of the hood of several 

 specimens in line with the median groove before mentioned. This may be an indication of an 

 originally less completely fused condition of the hood tentacles. 



Below the hood are eighteen digital tentacles upon each side. Including the tentacles 

 forming the hood there are thus nineteen upon each side, a number from which I found no varia- 

 tions in any of sixty -six specimens. Hut that variations in the number do occur is proved by the 

 fact that Humph and Owen each counted twenty tentacles upon a side, while Valenciennes 

 found only eighteen upon a side in his specimen. In another specimen Owen found only seven- 

 teen tentacles upon one side, while the number was normal upon the opposite side. 



The sheaths of the digital tentacles are fused to each other except for a distance of about 

 half an inch at their tips. The exterior of the cephalic sheath is made rough by the projecting 

 angles of the tentacle sheaths, and by this means the course of the individual sheaths can be 

 followed to a certain degree. The internal surface, on the other hand, is perfectly smooth except 

 at one. point. The exception is possessed by the female only. Upon the outside of the cephalic 

 sheath are seen four tentacles which are so much smaller than the others that they do not begin 

 to reach the anterior edge of the sheath. 



The tentacle next the hood on each side possesses a much larger sheath than the remaining 

 lateral tentacles, the increase in size being especially expressed in breadth. The sheath is 

 considerably flattened and overlaps the next lower tentacle sheath as it is itself overlapped by 

 the edge of the hood. It is usually fused along nearly its entire length to the ventral side of the 

 hood, leaving a crease where the edge of the hood projects. (Fig. 1; Fig. 2, DT 2 .) Its outer 

 surface bears papilla? like those upon the hood. Normally the tip of the sheath is entirely free, 

 but occasionally its sheath and the hood are so closely fused that no line of demarcation can be 

 observed, the tips of the sheaths being included in the fusion. Rarely other more lateral 

 tentacles may be included in this close fusion, so that the area of the hood may be considerably 

 increased. 



The remaining tentacles present no differences beyond those of size and shape which we may 

 expect to find. The tips of the sheaths where their shape is unaffected by fusion are roughly 

 triangular or quadrangular. Usually one of the angles is turned outward, and this may be 

 continued as a distinct ridge nearly or quite, to the posterior edge of the cephalic sheath. 



At the sides of the head the posterior part of the cephalic sheath falls away abruptly to the 

 level of the nuchal region. (Fig. 1.) These posterior faces are quite smooth. From the hood they 

 slope first down and back, then, from the level of the lower edge of the eye, they slope down 

 and forward. The faces become narrower as they pass downward and gradually disappear upon 



