124 MEMOIKS OF THE NATIONAL ACADEMY OF SCIENCES. 



upon the flap coverincj the tip of the fourth tentacle. Upon its .surface numerou.s pore.s, the 

 openings of tulnilar glands, are ea.sily seen. 



The bulk of the spadix is composed of the cirri of the lirst, second, and third tentacles. 

 The fourth tentacle may almo.st be said not to enter into the formation of the spadix, so small 

 is it compared with the mass formed by the other tentacles. (Fig. 19.) 



The cirri of the spadix are not of equal length, like tho.se of the antispadix. Neither their 

 bases nor their tips are at the same level. Text-iigure o is intended to show the relative length 

 and position of the cirri of the spadix by means of diagrammatic sections of four regions of the 

 organ. In A, a section through the tip of the spadix, the fourth cirrus does not appear. In 

 section B, taken a quarter of its length from the tip of the spadix, the tip of the fourth cirrus 

 is cut through. In section C, taken just below the middle of the spadix, the second cirrus has 

 ended, and in the position it occupied we see only bundles of muscle fibres passing from its base 

 in the sheath of the spadix. The fourth cirrus is no longer completely shut in by the flap of the 

 padix sheath. In section D, taken through the base of the spadix, the third cirrus has disappeared 

 in like manner as the second. In spite of the disappearance of two cirri the .spadix has increased 

 in .size continually as we have approached its ba.se. This is accounted for almost entirel3^ by 

 the great increase in the size of the first cirrus alone. But the fourth cirrus has also increased in 

 size and is, at the base, included within the sheath of the spadix. Strands of muscle from the 

 bases of the second and third cirri also form a portion of the organ. The fourth cirrus is still 

 fj-ee from the sheath of the spadix back of where the tissues of the first cirrus and the sheath 

 unite. Finally, the tissues of all the tentacles are united to form a solid base for the spadix, 

 which is firmly attached to the posterior region of the cephalic sheath and the cartilage. 

 Let us now turn to more detailed descriptions of the separate portions of the spadix. 





THE SHEATH OF THE Sl'ADIX. 



This somewhat indefinite nam(> is nevertheless convenient, and for this reason it seems 

 appropi-iate and worth retaining. From the previous description and diagrams it will be noticed 

 that in the anterior region of the sheath it is composed of the sheaths of the tentacles alone, 1)ut 

 in its posterior or liasal part it includes the muscle tissue extending from the bases of the 

 second and third tentacles. 



Sometimes slight longitudinal grooves upon the outside of the sheath indicate the position 

 and boundaries of the cirri within. (Fig. 13.) Upon the outer side of the spadix, i. e., the side 

 turned away from the mouth, is a deeper groove which receives the fourth cirrus. The anterior 

 end of this groove is hidden by the large flap developed on this side from the sheath of the 

 spadix. This flap, extending along the entire length of the .spadix, is widest near the tip of the 

 organ, while it becomes a mere ridge near the base. The free edge of the flap is thin and evenly 

 curved. It is about twelve millimetres in width at its broadest part. 



Examination of the sheath with a lens reveals minute pits upon the surfaces near its tip, 

 numerous upon the outer surface and less so upon the inni>r surface. Except for these and the 

 glandular area upon the outer side the sheath is quite smooth. 



The sheath is compo.sed of an external layer of longitudinal muscle fibres, and of inner circular 

 fibres ringing each cirrus cavity. It is covered by a single-layered epithelium, composed of 

 slender columnar cells, the basal halves of which are occupied by elongate, oval nuclei. Goblet 

 cells, filled with granular .secretion, are found in exceedingly great numbers in the epithelium of 

 the external surface of the anterior portion of the sheath, Init in small numbers upon the basal 

 portion. The pits and short grooves upon the anterior portion of the sheath, noticed under the 

 lens, are lined mostly by mucus-secreting cells and serve the purpose of increasing the secretory 

 surface. The epithelium upon the inner surface of the sheath is very similar to that upon the 

 outer surface, except that the cells are shorter, the nuclei nearly round, and the goblet cells very 

 rare. Indeed, the latter are only present near the edges of the cavities occupied l\v the cirri. 



Immediately under the epithelium is a di>i-niis of felted fibrous and elastic connective tissue. 

 In the thinnest portions this, containing a few muscle fibres, forms the entire sub.stance of the 



