146 REPORT ON THE PENNATULIDA. 



As already noticed, and as shown in Figs. 1 and 2, the leaves are 

 not all of the same size; the largest, which have a length of ^in. in the 

 female specimen, being situated in it a little below the middle of the 

 rachis, but in the male specimen a little above this point. This 

 difference in the position of the largest leaves causes a characteristic 

 difference in the general shape and appearance of the two specimens ; 

 a difference which may possibly prove to be an external sexual 

 distinction, though we have as yet no further evidence in support of 

 this suggestion. 



The number of component polypes varies, as already noticed, with 

 the length of the leaf ; the maxiinum number, fifteen in the male and 

 twelve in the female specimen, being only found in the leaves about 

 the middle of the series. 



The base of each leaf extends very nearly, but not quite, to the 

 mid-dorsal line of the rachis (Fig. 3). The most dorsally situated 

 polype of each leaf, which we have seen is also the shortest, usually 

 projects over towards the opposite side beyond the middle line (Fig. 3), 

 and these dorsal zooids projecting across the middle line alternately 

 from either side give rise to the zigzag appearance seen down the mid- 

 dorsal line in Fig. 1. 



Concerning the mode of development of the leaves we have noticed 

 the following points. The most dorsally situated polype of a leaf is 

 very often decidedly smaller than the other polypes, and this is especially 

 the case in small and apparently young specimens. Towards both top 

 and bottom of the rachis the leaves are smaller, and consist of fewer 

 polypes than in the middle portion ; but between the top and bottom 

 leaves there is this difference : in the top leaves all the polypes are 

 large, fully formed, and of equal size ; but in the leaves at the bottom 

 of the rachis all the polypes are below the average size, the dorsal ones 

 are the smallest of all, and may be rudimentary, while the more 

 ventrally situated ones gradually increase in size, and the largest of all 

 is the most ventral one. 



From these facts we conclude (1) that in the development of each 

 leaf the ventral polypes are formed first, and the others in succession, 

 one above the other, so that the the ventral polype of a leaf is not 

 only always the longest but also always the oldest, while the most 

 dorsally situated one is both the shortest and the youngest. Each 

 polype is thus at the time of its first appearance the most dorsal 

 one of the leaf to which it belongs, and becomes subsequently pushed 

 down towards the ventral surface by the formation of younger ones 

 still more dorsally situated, space being provided for these new 

 ones by the lateral growth of the rachis itself. This view also explains 

 the fact, noticed above, that the most dorsal polype of each leaf 

 projects across the middle line over to the opposite side, this being the 

 only direction in which its growth is not opposed by neighbouring 

 polypes. (2) That the uppermost leaves are the first-formed ones, 

 and therefore the oldest, and that new leaves are formed at the 

 bottom of the rachis below the previously-formed ones, the lowest 

 leaf being always the youngest. 



