2U 



HARDWICKE'S SCIEN CE-GO S S IP. 



THE SPONGEOUS ORIGIN OF ELINTS. 

 By Peed. Kitton. 



THE position of the Sponges seems to be midway 

 between the Amoebas and the Poraminifera, 

 the former consisting entirely of sarcode, and 

 without any kind of spicula or external shell ; and 

 the latter possessing a shell composed of one or 

 more chambers. The sponges, although destitute 

 of an external shell, possess a keratose skeleton, 

 strengthened in the majority of cases by calcareous 

 or silicious spicula. 



The vital and therefore the most important part 

 of the sponge is the sarcode, and with your per- 

 mission I shall endeavour to give as lucid a descrip- 

 tion as possible of the important part performed by 

 this ma^^erial ; and perhaps the best idea of it can be 

 obtained by soaking a piece of isinglass in water. 

 The living sarcode, like the softened gelatine, is semi- 

 pellucid, varying in colour from external causes, 

 and during life insoluble in water. 



The absence of special organs in the Amoebre, 

 Sponges, &c., clearly indicates that the power of 

 assimilating nutriment is possessed by this material, 

 and identifies it with the sarcodous system, covering 

 the digestive surfaces of animals. We can trace the 

 presence of this wondrous matter from the highly- 

 developed mammal to the humble amoeba ; other 

 organs may become obsolete, and at last we find 

 that sarcode alone remains. And this apparently 

 inert, shapeless, structureless mass of jelly is en- 

 dowed with the power of producing those elegant 

 forms known as polycystina and foraminifera, or, as 

 is the case with the sponges, the multitudinous 

 varieties of spicula (Bowerbank figures and describes 

 between two and three hundred distinct forms of 

 spicula, and does not then describe all the forms), 

 and even builds up with silex abstracted from the 

 waters a silicious skeleton of surpassing beauty, 

 as seen in the Euplectella and Dactylocalyx, or 

 elaborates long bundles of silicious fibres, as in 

 Hyalonema and Pliaroncma. 



Another substance found in sponges, and that 

 with which we are best acquainted, is keratode, or 

 horny fibre : this, like the silicious framework just 

 alluded to, is invested with sarcode. In some genera 

 (as in the sponges used for domestic purposes), 

 no spicules are formed; in others the spicules 

 predominate. 



As we find among the foraminifera certain forms 

 which do not secrete a shell, but form one by 

 gluing together minute grains of sand, so may we 

 also detect certain species of sponge in which the 

 skeleton has neither silic nor solid keratose, but 

 is composed of grains of sand inclosed in a thin 

 keratose covering. 



I must, however, now call your attention to the 

 protoplasm found in layers on the ooze at the 



bottom of the sea. Those of us who attended the 

 Biological section of the British Association in 

 1868, will remember an interesting paper by Pro- 

 fessor Huxley on Bathybius, in which he described 

 thi5 sarcodous layer, and the remarkable forms 

 occurring in it, known as coccoliths and cocco- 

 spheres. These forms may be found in chalk, clearly 

 indicating that the bottom of the sea, during the 

 Cretaceous period, was also covered in places with 

 protoplasm. I must ask you to bear this fact in 

 mind, as I think we shall find that it has an 

 important bearing on the theory of the Spongeous 

 Origin of Chalk Plints. 



If a thin chip or section of flint is submitted to 

 microscopic examination, sponge spicules in more or 

 less abundance will invariably be seen ; mixed with 

 these will be found casts of the interior of the 

 chambers of foraminifera, fragments of polyzoa, and 

 small molluscous shells. 



When a recent .'sponge is examined, similar or- 

 ganisms will be seen entangled in the reticulated 

 skeleton. A cretaceous flint, like silica obtained by 

 dialysis, is non-crystalline, breaks with a distinct 

 conchoidal fracture, is singly refractive, and there- 

 fore is not affected by a polarized beam of light, in 

 this respect resembling silica taken up by undoubted 

 animal or vegetable organisms. I may, perhaps, be 

 reminded that the cuticle of the Dutch rush and 

 the stellate hairs of Deutzia, both of which are 

 silicious, do polarize, and exhibit brilliant colour 

 when examined by light in that condition ; but this 

 is not in consequence of their silicious nature, but is 

 due to the presence of a membraneous film in- 

 vesting the cuticle or hair. If a piece of Equisetum 

 or Deutzia is boiled in sulphuric acid, and then 

 decarbonized with chlorate of potash, a display of 

 colour will no longer be visible. The shells of the 

 polycystina, sponges, spicules, and the diatomaceoe 

 are all singly refractive. 



The base of silica is silicon. Silica under certain 

 conditions is soluble in water to a considerable 

 extent. Waters holding silica in solution, — that is to 

 say, in any large quantity of it, — are now extremely 

 rare ; the Geyser and llykum in Iceland, and the 

 Pennakoon and Loongootha in India, are the best 

 known. An analysis of a gallon of the Geyser 

 water showed 31"50 of silica. It is highly probable 

 that silica was present in larger quantities in the 

 earlier epochs of the world. This, however, is not 

 a question of much importance, as we know that it 

 exists in a soluble form, and is eliminated, often in 

 great abundance, by various organisms. I need 

 only refer you to the Alcyoncellum, Hyalonema, 

 Pharonema, and other silicious sponges, as evidence 

 of that fact. 



The presence of silica in a state of solution being 

 an ascertained fact, there is nothing improbable in 

 the hypothesis that sponges should have formed 

 the nuclei of these flinty concretions : the silicious 



