326 PROFESSOR ALLMAN. 



of lierbivora, inasmucli as this latter, at the period of the com- 

 mencement of its deposition, exists in a precisely similar form. 

 This explanation of its nature also serves to remove the 

 difficulty which many persons have felt in accepting Wal- 

 deyer's supposition that it was cornified ; it certainly did 

 appear exceedingly improbable that a part of the enamel 

 organ should undergo this metamorphosis whilst the re- 

 mainder was calcified ; but if we regard it as tissue Avhich 

 has undergone a peculiar modification preparatory to, or 

 early in the process of, calcification, the difficulty disappears, 

 for such tissues bear no small resemblance to horn in their 

 behaviour, and are to be found, as has been already men- 

 tioned, elsewhere on the border of calcification. 



Notes on Noctiluca. By Prof. Allman, F.R.S., F.L.S., &c. 

 (With Plate XVIII.) 



The occurrence some years ago of Noctiluca miliaris in 

 great abundance during the autumn months on the southern 

 shores of Ireland afforded me an opportunity of studying 

 the structure of this singular little animal. The following 

 paper contains an account of the observations then made. 

 These are for the most part confirmatory of the results arrived 

 at by other observers, while in some respects they serve to 

 supplement the researches of those zoologists who have 

 already made Noctiluca a subject of study. 



The form of Noctiluca miliaris is nearly that of a sphere 

 so compressed, that while in one aspect (fig. 1) its outline 

 when projected on a plane is nearly circular, it is in the aspect 

 at right angles to this (fig. 2) irregularly oval. Occupying 

 about half of one of the meridians of this compressed sphere 

 is the long narrow entrance {a) into a deep depression of the 

 surface which I shall designate as the atrium {h), while along 

 the opposite meridian there extends a very slightly elevated 

 ridge (figs. 1 — 3, c) of a firmer consistence than the rest of the 

 body, and having somewhat the appearance of a rod embedded 

 in the walls. This commences with the appearance of a bifur- 

 cation close to one end of the entrance to the atrium, and 

 terminates abruptly after running over about one third of the 

 circumference of the body. Busch^ appears to have been 

 the first to call attention to the presence of this rod-like struc- 

 ture, which was again accurately described by Dr. Webb.- 



' 'Beobacht. iiber Anat. und Eutwickl. einiger wirbellos. Seethiere,' p. 103. 

 Berlin, 1851. ° 



" 'Quart. Journ. Mic. Sc.,' vol. iii, 1855. 



