108 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



the body is attached to the shell along three aponeurotic bands, two ventral and one dorsal, which 

 extend between the ends of the areas of attachment of the shell muscles. (Fig. 1.) 



The division of the body of the Nautilus into cephalic-, nuchal, and visceral regions is not as 

 distinct as it is in many dibranchiates. At times it proves convenient to recognize such divisions, 

 although they are entirely without morphological importance. For this reason we may consider 

 the cephalic region to be that which bears the tentacles, buccal mass, and eyes. A section through 

 the body just back of the eves would also pass just back of the central nervous system. The 

 nuchal region is that of less girth, which extends between t lie posterior edges of the cephalic 

 sheath and the attachment of the mantle. Accordingly, this bears the funnel and its crura, the 

 reproductive orifices, and the mantle. The visceral region comprises the remaining posterior 

 portion of the body, the wall of which we have already noted as thin and semitransparent. 



THE TENTACLES. 



A. — Digital Tentacles. 



For the purposes of description each tentacle will be considered to consist of two parts — a 

 cirrus, which is the active part of the organ, and a sheath which forms a protection for the 

 cirrus. The sheath of certain tentacles is lacking (or undifferentiated), the cirrus, never nor- 

 mally. In this application of the terms we need not take into account the probable morphological 

 importance of the sheath. 



The digital tentacles include those tentacles which form the cephalic sheath and hood. Each 

 is composed of two parts — a retractile (or extensible) adhesive cirrus, and a tough, thick-walled, 

 more or less rigid sheath into which the cirrus may be entirely withdrawn. (Fig. 1, C; Fig. 4',»: 

 Fig. 50.) This is the essential structure of not alone the digital tentacles, but all the tentacles 

 with which a Nautilus is so generously supplied. Whatever differences there are. they are 

 modifications of this plan. 



The digital tentacles are symmetrically arranged upon each side of the head, according to 

 the diagram presented in text-figure 2, CS, p. 116. A careful examination - of more than riftv 

 specimens has led me to make this statement in the face of other statements denying any regularity 

 of arrangement of these tentacles. Whether each individual tentacle, as determined by the inner- 

 vation, always occupies the same identical position is more than I can assert, because of the 

 extreme difficulty of satisfactorily following the nerve to each tentacle. However this may be, 

 the arrangement of the tentacles follows a definite plan from which variations were found in only 

 five specimens out of fifty-one examined at one time. * Except in one case the variation occurred 

 upon one side only. I see no especial morphological importance to be attached to this arrange- 

 ment; nevertheless, the fact is interesting and its knowledge may sometime be helpful when the 

 development of Nautilus comes to be studied. 



As has already been mentioned, the cephalic sheath is formed by the fusion of the sheaths of 

 the individual digital tentacles. The hood, which forms the entire dorsal part of the cephalic 

 sheath, is itself composed of the enormously developed sheaths of two tentacles. (Fig. 1, Ho: 

 Fig. '2, Ho.A.) The hood is roughly triangular in shape. It is thickest in its middle posterior 

 part, sloping from here to thin edges anteriorly and laterally. It presents three superficial fascia' 

 (Fig. 2); a dorsal about an inch in width sloping downward and forward to the anterior edge, 

 and two lateral which slope from the middle fascia to the lateral edges. The dorsal fascia main- 

 tains an equal width throughout its length — this varying in different specimens between 18 

 and 25 millimeters. Accordingly the anterior edge of the hood is approximately straight and 

 not pointed. The lateral fascia 1 , however, are widest opposite the posterior end of the mid- 

 dle fascia, gradually narrowing to a point anteriorly, and to a blunt rounded extremity pos- 

 teriorly which overlies the umbilicus of the shell. The posterior surface of the hood is pressed 

 closely against the involution of the shell, and following its shape is deeply concave. The postero- 



*The tentacles of a dozen other specimens since examined have conformed to the same plan. 



