37 



Plate VIII. fig. 6 is a figure of this polyp-skin in a species in which 

 there is considerable regularity in the arrangement of the polyps. 



The polyp-cells are lodged in the substance of the under-skin, 

 extending in the plane of its thickness the whole depth. They 

 lie at distances from one another about equal, or rather more, to 

 their own diameter. In the V. simplex their figure of arrange- 

 ment has a degree of regularity partaking of the quincuncial, a 

 figure so characteristic of the Polyzoa. In the convoluted varie- 

 ties it is (perhaps apparently only) less generally regular. 



What gave a vastly greater importance to and safety in reliance 

 on the facts thus observed, both as to the polyp-cells and the 

 moveable processes, was this : that I found that each of them was 

 only present in certain cases, always absent in others ; and that 

 those places where both were always absent were constant*. 

 Neither polyp-cell nor process is ever found on any part of the 

 root ; nor in those varieties characterized by a head, to which I 

 have already alluded, is either ever found on any part of that 

 headf. It will be felt to be necessarily consistent with the 

 character which I have shown to mark the roots of the Ventri- 

 culidaB that neither polyps nor processes should be found there. 

 I consider it to be the characteristic of the head-bearing Ven- 

 triculidse that those heads are apolypous. The polyp-skin and 

 under-skin are perfect on those heads, exactly the same in all re- 

 spects, excepting in the entire absence of polyp-cells and processes, 

 as the polyp-skin and under-skin covering the rest of the body. 

 The examination of those heads serves therefore as an important 

 test. 



Every fact observed in less perfect specimens is consistent with 

 the important results thus obtained. 



By a section most happily made, I have obtained (PL VII. 

 fig. 13) a transverse view of a few polyp- cells in flint J. The 

 section is somewhat oblique, which it is almost impossible in such 

 minute structure to avoid, and hence the fibres appear irregular 

 owing to those which lie in really different planes striking the eye 

 in the same focus. A slight variation of the focus shows that 

 the - under-skin is closely attached to the polyp-skin, rising 



* I have sometimes thought that I could detect obscure traces of cells under 

 the upper part of a root on the body of the Ventriculite. This would be 

 quite consistent with well-known phsenomena, and I will only refer to Milne 

 Edwards' paper, already cited, p. 211, for a very apt illustration of this 

 obliteration of cells and overgrowth of root-fibre. 



f The existence of this head-bearing group is another strong evidence of 

 the unity and entirety of the whole polypiferous mass. 



J This section was obtained long before I had satisfied myself of the ex- 

 istence and character of the polyp-cells ; and it was only when I had satisfied 

 myself of those facts by other means that I re-examined this specimen and 

 found its entire consistency with those inferences. 



