134 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



OCULAR TENTACLES. 



There arc four of these tentacles, two .springing from near the base of the stalk of each 

 eye. The eves, it will be remembered, are attached to the sides of the head in the angle formed 

 by the posterior edge of the lateral portion of the cephalic sheath and the projecting auricles of 

 the hood. The ocular tentacles arise above the level of attachment of the eyes, one in front of, 

 and the other behind, each eye (Fig. 1, O' and O"; Fig. 2, O'; Fig. 3, O"). Accordingly, they 

 are distinguished as the preocular and the postocular tentacles. 



The preocular is situated upon the base of the cephalic sheath, immediately back of the 

 posterior corner of the sheath of the second digital tentacle, and in the angle formed by the 

 projecting sides of the hood and the lateral walls of the head. Its sheath forms a portion of the 

 posterior part of the cephalic sheath, only a few millimeters of its tip being free. 



The postocular arises above the posterior edge of the eye in the angle made by the hood and 

 the sides of the head. It stands straight out from the sides of the head, having a free sheath 6 

 to 10 millimeters long. The preocular is directed outward and forward, the postocular outward 

 and slightly backward. Both tentacles are well protected by the projecting sides of the hood. 



The cirri of the ocular tentacles are in their general features like those of the digital tentacles, 

 but under the general similarity are most important differences of structure as well as of function. 

 They are oval, lacking the angles of the digitals (Fig. 65). The annular grooves are very deep 

 upon the anterior sides, and this is especially noticeable near the tips of the cirri (Figs. 64 and 

 63). Owen (1832, p. 14) well describes these cirri as "in reality composed of a number of flat- 

 tened circular disks appended to a lateral stem." The closely pressed ridges project from the 

 base to the tip of the cirrus like the lateral plates of a cephalopod gill. 



Observations made by Willey and published in the fortieth volume of the Quarterly Journal 

 of Microscopical Science are well worth quoting in this connection : 



"The occurrence of a special tentacle in front of the eye and another behind the eye in Nautilus 

 is well known. These tentacles resemble the large number of remaining tentacular appendages 

 in being ringed and also in being retractile within sheaths, but differ from them in almost every 

 other respect. In the first place, most of the tentacular appendages of Nautilus have essentially 

 an adhesive function, to which is related a prehensile function. They are employed for seizing 

 hold of food and for attachment to surfaces 



"'It will not be surprising to learn that the adhesive tentacles are not ciliated; but it is neces- 

 sary to mention this negative fact, because the preocular and postocular tentacles are ciliated. 

 On the side corresponding to the suctorial ridges of the adhesive tentacles the annulations of the 

 preocular and postocular tentacles form deep grooves, between which the ridges project as promi- 

 nent lamellae. The upper and lower surfaces of the lamella' and the bases of the grooves are 

 covered with vibratile cilia. There can be but little doubt that the preocular and postocular 

 tentacles of Nautilus represent tentacular processes, homologous with the adhesive tentacles, 

 which have been modified to serve an accessory olfactory function. We will therefore speak of 

 them as the olfactory tentacles, in contrast to the adhesive tentacles 



"The olfactory tentacles .... when extended stand out from the body nearly at a 

 right angle, the preocular tentacle being directed slightly forward and the postocular tentacle 

 usually tending backward. The ciliated olfactory lamellae are directed strictly forward. 



"In the living Nautilus the olfactory tentacles otherwise offer a strong contrast to the 

 adhesive tentacles by their almost uniform white color. When examined under the microscope 

 there is found to be a little brown pigment in the annulations and at the edges of the lamellae, 

 but when viewed in toto under water the general color effect is white. 



" Moreover, the adhesive tentacles can be touched without necessarily being retracted, but at 



the slightest contact with a foreign body the olfactory tentacles are instantly retracted within 



their sheaths. The presence of accessory olfactory tentacles in Nautilus can, I think, be related 



to an essential bionomical difference between the existing Tetrabranchiata and the Dibranchiata. 



'Nautilus finds its food chiedy by the sense of smell, while it is a matter of more or less 



