MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 129 



suggests that it is covei'ed by a rough cuticle. This, however, is not the case, as we shall 

 presenth' see. 



The middle part of the cirrus, where the transition from the oval to the tlattened shape takes 

 place, is triangular in section for a distance. (Fig. 59.) The inner side of the cirrus forms the 

 short side of the triangle. The dorsal and ventral suiface« of this region liear their characteristic 

 structures. The inner surface is smooth and muscular. 



In general the structure of this cirrus is so like that of the second that no detailed description 

 is necessary of any part except the pits. In Fig. 59 we have a transverse, slightly oblique section 

 of the third cirrus, and in Fig. 60 a longitudinal section of the same cirrus, both drawn with the 

 aid of the camera lucida. Fig. 62 shows a longitudinal section through the center of a pit 

 magnified to 34 diameters. The pits are simple cavities 1 millimeter in depth, into which a 

 fleshy tongue projects from the posterior side as the animal is in its swimming position. Suppos- 

 ing the cirrus to be placed tip upward, the tongue projects from the floor of the cavity. The 

 cavity of the crypt forms only a narrow Assure about three sides and the edges of the fourth side of 

 the tongue. This is thus attached to the wall of the crypt along a narrow median region. (Figs. 

 59 and 60.) The tongue is stronglj^ muscular, the muscle fibres mostly extending from the base 

 toward the tip. The tissue between the crypts is largely muscular: all the muscle fibres, both of 

 the tongues and of the intermediate tissues, are much finer than those making up the body of the 

 cirrus. Many connective tissue nuclei are scattered among the muscle fibres. 



Beneath the epithelium of the sides of the crj-pts and their tongues is a clear layer of a peculiar 

 structure. (Fig. 62.) This layer does not extend quite to the bottoms of the crypts, but commenc- 

 ing near here, becomes gradually thicker and thicker as the openings of the crypts are approached. 

 The clear layer is especially thick upon the anterior edge of the ciypt, i. e.. upon the edge toward 

 the tip of the cirrus. Undei' low powers the layer appears homogeneous and structureless, but 

 high powers of the microscope reveal numerous and exceedingly fine fibres in it. No nuclei 

 whatever are found in the layer. At the extreme outer edge of the layer, immediately under 

 the epithelium, no fibres at all, nor any other structural element can be distinguished. Passing 

 inward, the fibres are progressively more and more distinct. Along the inner edge of the layer, 

 next to the muscle, is a distinct layer of small nuclei which appear like coimective tissue nuclei. 

 It is this clear layer which gives the surface of this portion of the cirrus its hard i-ha racter. It seems 

 to form a supporting tissue or sort of exoskeleton for the other tissues of the crypts, or perhaps 

 for the cirrus as a whole. 



The surface of the cirrus is covered with a single laj^ered epithelium. Those portions of 

 the cirrus not occupied by crypts are covered by an epithelium similar to that of the second 

 cirrus. In the region of the crypts it is considerably modified. In the deepest parts of the 

 crypts the cells are of about the same proportions as those upon the dorsal side of the cirrus — 

 slender columnar cells, among which are numei'ous goblet cells. The number of secretory cells 

 in these regions is not at all remarkable. 



As the cells extend outward upon the sides of the crypts and their tongues, they gradually 

 decrease in length and increase in Ijreadth, until a veritable pavement epithelium is formed. 

 The outer portions of the sides of the crypts and all the area between their openings is covered 

 by an epithelium of this character. 



The function of the crypts is entirely unknown, and I do not see that we have facts of 

 any kind upon which to base even guesses as to the nature of their uses in the economy of 

 Nautilus. The nmiiber of glandular cells in the crypts is so small that, as Vatssiere says, the 

 latter can scared}' have been developed for the purpose of inci'easing glandular area. The 

 development of muscle tissue about the crypts, and especially in their tongues, together with 

 the development of a firm, hard layer upon their surfaces, indicates a considerable and important 

 activity for these structures and foi- the ciri'us as a whole as weU. 



FOlIRTn C:II!KUS OF SPAIIIX. (fIG. 19.) 



As has been said, this cirrus is almost unmodified. It is the smallest of the group and is 

 nearly hidden by the flap upon the outer side of the spadix. It is scarcely longer than the 



