VIRGULARIA MIRABILIS. 68 



point of origiu of new leaves is the bottom of the rachis, and in both 

 we have the same arrest of development after reaching a certain 

 limit. 



In Virfiuhiria, however, the successive leaves tend to separate from 

 one another to a far greater extent than they do in Peiiiiatula, while 

 in the latter the lateral growth of the individual leaves is very much 

 greater than in I'injularia. Another point of difference lies in the 

 fact that while in Pennatula the several polypes of a leaf are developed 

 successively, in Mrtiularia they appear simultaneously, the youngest 

 leaves having the same number of polypes as the oldest or most mature 

 oues. 



Concerning the calcified stem it is clear that it also must grow so 

 as to keep pace with the whole colony. Frora its extremely dense 

 structure and the very large proportion of inorganic matter it contains, 

 it seems very improbable that it can grow iuterstitially along its whole 

 length ; indeed, it appears almost certain that growth only occurs by 

 the addition of new matter, either at the ends or on the outside of that 

 which is already formed. If it be also true, as noticed previously, that 

 the top of the stem normally projects bare for a short distance above 

 the top of the rachis, then it is clear that the stem can only grow in 

 length by addition to its lower end i.e., that it is continually being 

 pushed up, as it were, through the rachis from below, and that the 

 growth of the stem in length, though not in thickness, is independent 

 of that of the rachis. Increase in thickness is effected by the deposi- 

 tion of successive laminae one outside another by the soft tissues of 

 the rachis and stalk in contact with the stem. 



Though the several polypes of each leaf come into existence simul- 

 taneously, and in the smallest leaves the number of polypes is the 

 same as that in the most fully developed ones, yet we find that from 

 the time of their very first appearance there is a gradual increase of size 

 as we pass from the most dorsal polype of a leaf towards the most 

 ventral one. This is shown clearly for the fully developed leaf in 

 Fig. 5, and for tlie early stages of development in Fig. 6. 



This difference in size between the dorsal and ventral polypes of a 

 leaf might be explained, so far as the adult leaves are concerned, by 

 the greater freedom and range of action, and consequent greater chances 

 of obtaining food possessed by the ventral as contrasted with the 

 dorsal polypes ; but this explanation would hardly account for the 

 dift'erence in size being so marked in the very earliest stages of their 

 development. We are disposed to think that the true explanation is 

 that in the ancestral forms either of V irriularia itself, or from which 

 Virgidaiia was derived, the several polypes were, as in FiDiicuUna and 

 Fcnnutula at the present day, developed not simultaneously but 

 successively one above another, the ventral ones first ; and that though 

 Virgularia has lost this primitive character, and has acquired the habit 

 of developing all the pol}-pes of a leaf simultaneously, it has still 

 retained indications of its ancestral habits in the greater size of the 

 ventral polypes, e\en in their earliest stages. It is just possible that 



