of the Chalk. 187 



detected by aid of the microscope. I was then fully prepared to 

 find this part of my researches unsuccessful. 



A very minute examination, however, of my most delicately 

 preserved specimens at length threw some light even on this 

 point. I found that, in all the most marked varieties of form of 

 the Ventriculidae, in that membrane which I have described as 

 lying external to the central part of the polypidom and imme- 

 diately underneath the epidermis, and which membrane is above 

 distinguished as the under-skin, there were numerous spots, scat- 

 tered over the surface, which that membrane did not fill. The 

 size of these, though varying, as was to be expected, in different 

 species, does not do so very greatly, and may be stated as ranging 

 between the 100th and the 200th of an inch. These are arranged 

 in some species with more, in others with less regularity. A very 

 rigorous examination of such specimens, tested by sections, led 

 me, after a length of time, to the conclusion, that these vacuities 

 are the true polyp-cells*. I may observe, first, that they are 

 found in the most different species, including the so-called 

 Ocellarice ; secondly, that they correspond in mode of position 

 and character to the polyp-cells of many of the Polyzoa and espe- 

 cially of the Halodactylus diaphanus, differing only from the latter 

 in being circular, because implanted in a more strongly developed 

 and highly organized connecting medium or tunic which is found 

 regularly separating each one from its neighbours f, — a fact con- 

 sistent with the higher degree of organization of these animals, 

 and seeming again to point to the singleness of the entirety of the 

 whole polypiferous mass; thirdly, but not least importantly, that I 

 have since found specimens in which the polyp-skin is, through 

 the deposit of sulphuret of iron as already explained, well-pre- 

 served, in which specimens these cells are seen marked with full 

 as much distinctness in that polyp-skin as in any specimen of 

 Halodactylus diaphanus or of many other recent polypifers. On 

 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- 



* It is important to observe, as again showing the absence of any analogy 

 with the Alcyonium, that not in these polyp-cells or on any part of the sur- 

 face of any of the Ventriculidae is there any trace of those stellate figures 

 which distinguish the surface of the Alcyonidae, and indeed of all the Aste- 

 roida, and traces of which would necessarily have been preserved had they 

 or their related objects ever existed. 



f See note, p. 186. 



