MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. Ill 



usual, bat not a constant, feature is a shallow Longitudinal groove occupying the middle of the 

 inner face of the cirrus. (Fig. 4'.'.) 



The annular grooves are much deeper upon the inner side than upon the outer sides, and as a 

 consequence the alternating ridges are also much more prominent upon the inner side. (Fig. 50.) 

 To use a comparison, the cirri are somewhat like high piles of thin, flat disks, fastened together 

 through their centers. This comparison, however, applies closely to the terminal third or half 

 of the cirrus only. Upon the proximal part, which is always retained within the sheath, the 

 annular grooves quickly disappear on the outer surfaces. The grooves persist on the inner side, 

 gradually becoming fainter and fainter, until within a couple of centimeters of the proximal end 

 of the cirrus. The proximal end of the cirrus is usually perfectly round and perfectly smooth; 

 it tapers slightly to the base, which has about half the greatest diameter of the cirrus. The 

 terminal disks are broken ofl' with great case: the query arises if regeneration of sonic sort does 

 not take place. The inner projecting part of each disk forms an adhesive organ, the structure 

 of which will presently be described in detail. 



The tissues of the cirrus are continuous at its base with the tissues of its sheath. The 

 sheaths of the larger tentacles have a depth of about 6 centimeters. The epithelium extends 

 from the surface of the cirrus upon the inner surface of its sheath, and over the margin of the 

 latter upon the surface of the body. The free portion of each sheath is usually angular and 

 slightly tapering. At its blunt extremity is a round, oval, or slit-like opening, through which 

 the cirrus projects. The cavity of the sheath frequently extends to near the cephalic ganglia — 

 much farther, therefore, than there is any external indication of the individual sheath. With 

 the exception of those two which form the hood, one sheath is very much like another. The 

 reasons for believing the hood to be composed of the greatly enlarged and closely fused sheaths 

 of two ordinary tentacles have already been given. 



The tentacle sheaths are composed of a dense felt of large, branched elastic fibres; its inter- 

 stices are occupied by white fibrous connective tissue fibres and nuclei. Small scattering bundles 

 of longitudinal and transverse muscle fibres traverse the sheaths. In places the longitudinal 

 muscles approach a regular arrangement. The amount of muscular tissue is, however, so small 

 that there can be but very little movement of the individual sheaths. The external surface of 

 the sheath is covered by a columnar epithelium, the cells of which measure 52/* by 4//-8/<; the 

 cells lining the cavity of the sheath are much lower, being only 24/< by o/i-i>/< in their dimen- 

 sions. Both inner and outer epithelia contain very numerous glandular cells. The oval nuclei 

 are situated in the lower half of the cells, but do not all lie in one plane. Larue capillaries with 

 endothelial walls are. surprisingly numerous in the tissues of the sheath, and here, as elsewhere, 

 the capillaries do not collapse when empty. But the vascular system is not completely closed. 

 The tissues of the sheaths are like a line mesh work, in the spaces of which the blood flows freely. 

 There is no dermis. 



The hood presents some modifications of this plan. The cells of the epithelium measure 

 36/< by -fyu-6//. Immediately beneath the epithelium is a dermis formed by a layer of particularly 

 closely woven elastic fibres which, on account of its density, is easily distinguished by the naked 

 eye from the underlying tissue, although it can not be readily dissected away. The dermis is 

 almost entirely destitute of muscular fibres and is penetrated in every direction by great num- 

 bers of vascular lacunas. But even in the hood the dermis is not an absolutely constant char- 

 acter. Throughout the hood numbers of capillaries with endothelial walls can be seen. The 

 capillaries are of large caliber and, curiously enough, remain open when empty, so that their 

 cross sections are circular, in the posterior part of the hood many large muscles run in various 

 directions just beneath the dermis. They form a layer which seems to have much to do with the 

 contraction of the posterior portion of the hood. The muscles become smaller and less numer- 

 ous toward the outer side and the deeper portions of the hood. 



Posteriorly the hood is concave and tits snugly around the involution of the shell. The 

 epithelial cells of this are smaller than those of the upper surface of the hood and of the remain- 

 der of the cephalic sheath, their dimensions being 28/« by 3/M>A«. The epithelial cells of this 

 surface as well as those of the upper surface of the hood are loaded with fine granules of a 



