REPOUT ON THE PENNATULIDA. 241 



EEPOET ON THE PENNATULIDA 



COLLECTED IN THE OBAN DREDGING EXCURSION 



OF THE BIRMINGHAM NATURAL HISTORY AND 



MICROSCOPICAL SOCIETY, JULY, 1881. 



BY A. MILNES MARSHALL, M.A., D.SC, PROFESSOR OF ZOOLOGY 

 IN OWENS COLLEGE, AND W. P. MARSHALL, M.I.C.E. 



(Continued from page 227.) 



PART III.— ViRGULARiA MiR-VBiLis. Lamarck. — {Continued). 



3. — The Polypes and Zooids. — 



The general arrangement of the leaves is shown in Fig. 1 ; and the 

 leaves, together with the polypes of which they are formed, in Figs. 3, 

 4, and 5. In the Oban specimens each leaf is formed by the fusion of 

 seven to eight polypes, placed side by side, the number being constant 

 in all the leaves of any one specimen, but varying in different speci- 

 mens. The leaves are arranged strictly in pairs at the two ends of tlie 

 rachis, but about its middle often show slight irregularities, and may 

 even alternate with one another for some little distance. 



At the bottom of the rachis there is no trace of leaves or polypes, 

 but about an eighth of an inch higher up the leaves begin to appear as 

 small transverse ridges : they are at first very close together, and the 

 component polypes very small ; but passing upwards the polypes gradually 

 get larger and the leaves wider apart. Having reached their maximum 

 size and distance from one another, the leaves preserve these for some 

 distance, and then, towards the top of the rachis, begin gradually to 

 get smaller and closer together. 



In the Glasgow specimen, which has the rachis perfect at both top 

 and bottom, we have been able to measure accurately the number of 

 leaves in each inch length of the rachis. In the following table these 

 numbers are shown, as well as the " pitch " of the leaves at different 

 parts of the length. By " pitch " we mean the number of pairs of 

 leaves per inch length of rachis ; e.g., if in a given inch there are nine 

 pairs of leaves, then the " pitch " at that part of the rachis is 9 ; or 

 again, if in a given quarter of an inch there are four pairs of leaves, 

 this is at the rate of sixteen pairs in an inch, which is expressed by 

 saying that the " pitch "' at this part is 1(1. 



