31 



sponge-like mass, a firm and enduring body, not contractile*, 

 and with a peculiar structure for maintaining its normal shape. 

 The variety of actual form under which they are found being 

 very great, it must be particularly remembered also that any 

 essential character dependent on any apparent tubules, external 

 or internal, is most distinctly and unequivocally negatived. Were 

 other proofs wanting, this point is established by the fact that 

 such apparent tubules are found in some few special forms only, 

 while in the greater number of the forms of Ventriculidse all 

 marked, in common with those last-mentioned^ by those cha- 

 racters of structure above described, and which therefore must 

 necessarily constitute the essential and generic characters of the 

 whole group all trace of such tubules is absent. Some charac- 

 ters must therefore be found common to all these forms, or 

 common to many different forms, and the apparent absence of 

 which, if undetected in others, can be explained, before we can 

 hope to have got any hint pointing to the affinity of these bodies 

 with any recent forms. 



Having already investigated with every care the structure of 

 the central polypidom, while we know that in the actual integu- 

 ment of recent zoophytes very peculiar characters generally re- 

 side, it seems most proper to come back, for the purpose just 

 indicated, to the more careful examination of that which has 

 been hitherto only cursorily mentioned, namely, what I have 

 called the polyp-skin. But I have already stated how rarely 

 this exists in any degree of perfection : hence this investigation 

 is attended with great difficulties. And in truth the task is one 

 so difficult that I have frequently been on the point of relinquish- 

 ing all hope of being able to effect it, when the observation 

 of some character which had theretofore escaped attention has 

 renewed my hopes. But for the slitting-wheel and the micro- 

 scope, coupled with a careful study of the matrix, those cha- 

 racters must ever have escaped me. 



It will be convenient, and not in strict language incorrect, if we 

 distinguish the parts of which we have now principally to treat, 

 as the epidermis, cuticle, or polyp-skin already mentioned, and the 

 dermis, cutis, or under-skin, being the delicate membrane already 

 described as immediately underlying the polyp-skin. 



It will be remembered that, in the vast majority of specimens, 

 both in chalk and flint, the central framework of the polypidom 

 is alone preserved f. Both polyp-skin and under-skin, owing to 



* In addition to the points before named, it seems obvious that the con- 

 tractile theory requires that the animal should be strictly a single and not a 

 composite one. It cannot be conceived that a polypiferous animal should 

 have its polypiferous surface contractile over itself. 



t As Tubulipora patina would be, stripped of its cells, and as it is actually 

 described by Mr. Thompson. See Johnston's Zoophytes, p. 268. 



