62 REPORT ON THE PENNATULIDA. 



r 



A comparison of this with the other Oban specimens has led us to 

 a few general results of some interest. In the first place, we find that 

 in no one of the specimens is the pitch at any part less than 6 ; i.e., in 

 no part are there less than six pairs of leaves in an inch length of 

 rachis. We have already seen that the largest of the leaves are those 

 which are furthest apart, so that it would appear that, so far as the 

 Oban specimens are concerned, the limits of growth of the leaves are 

 reached when these have attained a distance from one another of J in. 

 In the table given above it will be seen that when this point has been 

 I'eached growth stops, and in the upper three inches the pitch remains 

 constant at the number 6 ; and the same thing applies to the other 

 specimens as well. Secondly, in five out of the seven specimens the 

 pitch at the upper end is 6, while in the remaining two specimens it is 

 8. If, as we have tried to show above, this number 6 is the limit, 

 and is only reached in those parts which have attained their full 

 growth ; i.e., in the parts at or about the middle of the entire colony 

 (c/. Fig. 1), then these facts would seem to show that the tops are usually 

 bitten off somewhere about, perhaps slightly above, the middle ; i.e., 

 that in the Oban specimens at any rate, the rachis, if complete, would 

 be something like double its actual length. Couceruiug the growth of 

 the leaves it is clear that, as in Pennatula, the seat of development 

 of the leaves is at the lower end of the rachis. 



Although the leaves get smaller and closer together towards both 

 upper and lower ends of the rachis, yet there is a great difference 

 between the two cases. At the upper end, just as in Peiinatulu, though 

 the leaves get smaller the polypes remain fully formed — a point we 

 have been able to confirm by an examination of the specimens taken 

 by Mr. Darbishire from the haddock's stomach. At the lower end of 

 the rachis ou the other hand, not only do the leaves get smaller, but their 

 component polypes get more and more imperfect, and at last (Fig. 6, dr) 

 become reduced to mere pit-like depressions of the surface. 



We conclude, therefore, that the topmost leaves are the oldest, the 

 lowermost the youngest : that the seat of development of the leaves is 

 the lower end of the rachis ; and that each actual leaf takes its rise 

 at this point, and gradually travels upwards as new leaves are 

 developed in succession below it ; that the colony grows along its 

 entire length, but that the limit of growth is reached, as already 

 explained, when the distance between successive leaves amounts to 

 ^ inch ; that this limit is never reached by the oldest or uppermost 

 leaves, which remain permanently sniall and close together, but that 

 as the colony gets older and older the pitch finally attained by the 

 leaves gradually diminishes until its final limit is reached. 



It follows from this that all the part of a Virgularia above the point 

 at which this final pitch is first attained has ceased to grow : and the part 

 below it is still gi'owing, but will cease to do so as soon as this limit is 

 reached. 



It will be seen that in many respects this mode of growth agrees 

 closely with that we have described in I'lnuaUda. In both cases the 



