AND HISTOLOGY OF SPONGES. 235 



to within tlie last few months I had always regarded the chambers 

 or burrows occupied by the Sponge as having been made by some 

 Annelid, and that the Sponge itself had nothing to do with the for- 

 mation of them, being only parisitical in them. Dr. Bowerbank had, 

 I believe, come to the same conclusion. From specimens that I have 

 prepared of portions of an oyster shell, which was full of the Sponge, 

 and also from the examination of a section of a shell of Haliotis 

 kindly lent to me by Mr. Kitton, of Norwich, chambered out in a 

 similar manner by another species, I have come to the conclusion 

 that no known Annelid could bore in the manner indicated in this 

 diagram (Fig. 6, Plate XVII.), where you will see that the ramifica- 

 tions are very numerous and in some cases very fine, and most of 

 them are more or less filled up with the Sponge or remains of the 

 same. 



Whether it is accomplished by a mechanical or chemical process, 

 or both, by the Sponge still has to be found out ; that it is the 

 action of the Sponge there can be, I think, little doubt, and I my- 

 self am rather inclined to think that both processes are brought into 

 action. 



As I intend to go more into this subject I shall reserve my reasons 

 for a future paper. 



Sponges seem to have existed in more abundance in earlier times 

 than they do now — more especially at the Cretaceous period — to wit- 

 ness, the Ventriculites and formation of Flints, which in the latter 

 either form the nucleus or the chief of those interesting and curious 

 substances. 



If a thin section of Flint be taken and examined you will generally 

 find, besides other forms, such as Foraminifera, spicules, and in some 

 cases entire reticulated Sponge structure, small spined bodies, which 

 have been termed Xanthidia, because they were at one time sup- 

 posed to be the sporangia of certain forms of Desmidiae, named 

 Xajithidium. 



From more extended observations they have been found to difi"er 

 much from their supposed homologues, answering more to the de- 

 scriptions we have of Sponge gemmules, or the statoblasts of some of 

 the marine Polyzoa. At any rate I think that they may now be 

 expunged from the vegetable kingdom. From what I have read and 

 seen I am not inclined to think that they are gemmules, but either 

 the statoblasts of some of the Polyzoa or analagous to the peculiar 

 bodies which have been found in some Sponges, and a description 



