52 ON THE STRUCTURE OF NOCTILUCA MILIARIS 



The anterior edge is not quite straight, but is formed by 

 two ridges, apparently of a harder substance than the 

 remainder of the outer membrane, which run up on the two 

 sides of the fissure, and unite, forming a very obtuse angle, 

 open anteriorly, in the base of the tentacle. 



The latter is a subcylindrical filament of 1-1 800th inch 

 diameter, more or less flattened, sometimes quite flat at its 

 free end, which is rounded at the apex. It is a little broader 

 at its base than elsewhere, and consists of an external struc- 

 tureless membrane continuous with the general investment, 

 and of an internal substance, which is so marked by transverse 

 granular lines, as very closely to resemble a primitive 

 fibril of striped muscle, I agree with Krohn that the 

 striation is not in the external membrane, as Quatrefages 

 states. 



From the bottom of the oral cavity a very delicate filament 

 (fig. 3), which exhibits a rapid undulating motion, is occa- 

 sionally pi'otruded, and then suddenly withdrawn. Krohn, 

 who first discovered this singular organ, considers that it plays 

 an important part in sweeping nutritive matters into the oral 

 cavity, and there can be little doubt that such is the case. I 

 would warn future obsei'vers not to be easily discouraged in 

 their search for this organ, I had sought for it in at least 

 fifty individuals without success ; and nothing but the firm 

 confidence in M. Krohn's accuracy, with which frequent 

 working over his ground has inspired me, led me to per- 

 severe until I had discovered it. Among the great numbers 

 of Noctilacce which I examined, however, I did not observe 

 half a dozen which presented a good view of the cilium. 



Under these circumstances, I do not comprehend how it is 

 that M. Krohn should have overlooked a very remarkable 

 structure which requires no such sharpness of vision as that 

 to which I have just alluded, I refer to an S-shaped ridge 

 arising close to the right extremity of the anterior oral margin 

 above described, and passing down on the right side of the 

 oral aperture to form its lateral and posterior boundary. 



This ridgo is horny-looking, and is considerably produced 

 in its middle portion into a tricuspid prominence (fig. 4 d\ 

 for which I know of no better name than a ' tooth.' This tooth 

 is about l-7000th in. high ; its middle cusp is stronger than 

 the other two, and bifid, while the posterior has a slight 

 pointed heel. I have never observed any movement in this 

 tooth-like body. 



Behind it the oral aperture narrows to inclose what may be 

 termed a post-oral space, and then widens again ; the eleva- 

 tions bordering this post-oral space are continuous with those 



