1U8 MEMOIRS OF TOE NATIONAL ACADEMY OF SCIENCES. 



the l)ody i.s attached to the shell along- three aponeurotic bands, two ventral and one dorsal, whicii 

 extend itetween 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 reg-ions 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 rea.son 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 eyes would also pass just back of the central nervous system. The 

 nuchal region is that of less girth, which extends between the 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 liody, the wall of which we have already noted as thin and semitransparent. 



THE TENTACLES. 

 A. — DiOlTAL TeNTA('I,ES. 



For the purposes of description each tentacle will b(> 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 more or less rigid .sheath into which the cirrus may be entirely withdrawn.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'.i; 

 Fig. 5(1.) 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 

 moditications of this plan. 



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

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

 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 liy the inner- 

 vation, alwavs occupies the same identical position is more than 1 can assert, l)ecause of the 

 extreme difficulty of satisfactorily following the nerve to each tiMitacle. However this may l)e, 

 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. * PLxcept in one case the variation occurred 

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

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

 development of Nautilus comes to ))e studied. 



As has already been mentioned, the i-ephalic sheath is formed by the fusion of the sheaths of 

 the individual digital tentacles. The hood, which forms the entire doi'sal 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 i)resents three superfiriul 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 vaiwing in different specimens between I.s 

 and 2,5 millin)eters. Accordingly the anterior edge of the iiood is approximately straight atid 

 not pointed. The lateral fascia", however, are widest oppf)site the posterior end of the mid- 

 dle fascia, gradually narrowing to a point anteriorly, and to a l)Iunt lounded extremity pos- 

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

 cioselv against the involution of th(> shell, and following its shape is deeply concave. Tli(> postero- 



*The tentacles of a ddzi-ii otlier specimens since examined have coiitunned to the same plan. 



