130 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



corresponding cirrus of the antispadix, but is of considerably greater diameter. The tip is 

 flattened. Elsewhere the cirrus is nearly round. Its lower portion is smooth, while the distal 

 half is more or less distinctly grooved. This cirrus has its origin at the very base of the spadix, 

 and is free for almost its entire length from the main portion of this organ. It possesses a very 

 short separate sheath which is not nearly as long as the sheath of the corresponding cirrus of 

 the antispadix. (Figs. 12 and 14.) This sheath is united to the base of the spadix sheath. The 

 base of the fourth cirrus passes under the base of the first cirrus to its outer side. Aside from 

 the reduction of the annular ridges, the appearance of this cirrus is not markedly different from 

 that of the digital cirri. 



The essential features in the structure of this cirrus are also almost the same as those described 

 for the digital cirri. (Figs. 54 and 58. ) The arrangement of the muscles is somewhat modified by 

 the great development of the transverse musculature, the strands of which are sometimes as large 

 as those of the longitudinal muscles. The regular radial arrangement of the latter is largely 

 lost. The epithelium of the annular ridges is not especially developed at any point, although 

 distinctly higher upon the ridges than between them. Large blood spaces run within the ridges 

 upon the inner side of the cirrus. They extend only a portion, one-half or less, of the distance 

 across the inner face of the cirrus. These spaces are lined with flat endothelial cells, and 

 communicate with veins lying deeper in the cirrus. They lie almost directly beneath the 

 epithelium. They are not present in the small ridges near the tip of the cirrus, appearing in 

 the tenth to the fifteenth from the tip, and increasing in size as the ridges increase in breadth 

 and height. 



The nerves of the spadix come off from the left pedal ganglion; those of the antispadix from 

 the right pedal ganglion. It is very difficult to trace the nerves into the individual tentacles, 

 although I have succeeded in doing this in a few instances. The four nerves to the cirri of the 

 spadix or the antispadix spring from the edge of the ganglion. They appear to belong to the 

 superior labial series, but being a little larger. A few special nerves pass from the ganglion 

 into the base of the spadix. One of these (Figs. 41, 30) forms an enlargement in the base of the 

 spadix, from which several small nerves pass into the surrounding tissues. 



In a less than half -grown male the spadix formed a very small, flat organ, resembling the anti- 

 spadix, which scarcely reached as far as the tips of the jaws. Vayssiere has already shown that 

 the cirri of the spadix are at first like those of the antispadix, and that they undergo modification 

 quite late in the life of the Nautilus. 



VAN DER HOEVEN'S ORGAN. 



Directly beneath the buccal mass of the male Nautilus is a peculiar organ discovered by 

 Van der Hoeven and since known by the name used at the beginning of this paragraph. (Figs. 

 8, 9, 10, 66, 67, 68, and 73.) To this organ an olfactory function has been ascribed, without any 

 evidence, to be sure, but apparently in accordance with a common custom of describing any organ 

 of Nautilus, the function of which is unknown, as an olfactory organ. The organ opens into a 

 pocket formed laterall}- by the labial ridges, ventrally by the cephalic sheath, and dorsally by a 

 ridge connecting the labial ridges. Into this pocket the anterior end of the organ projects freely. 

 (Fig. 8.) The walls of the pocket soon attach themselves to the organ and, being continued upon 

 its surface, form a tunic. This attachment takes place near the anterior end of the organ dor- 

 sally, but near the middle ventrally. (Fig. 9.) The organ is oval, the long axis lying parallel to 

 the long axis of the body, and is flattened dorso-ventrally. It is 'lb millimeters in length, 15 

 millimeters in width, and 10 millimeters in thickness. 



If the pocket, or atrium, into which the organ projects be opened as far back as the attachment 

 of its walls to the organ, we see that the anterior part of the latter is divided by a median longi- 

 tudinal Assure which extends from the ventral side nearly through to the dorsal side of the organ. 

 (Fig. 8, VF; Fig. 10.) The fissure does not extend on the surface quite as far back as the 

 attachment of the integument to the organ. Transverse and longitudinal sections reveal the 

 fact that the fissure just mentioned does, however, extend within the organ for some distance 



