MATERIALS FOR A MONOGRAPl! OP THE ASCONS. 469 



Materials for a Monograph of the Ascons.— I. On 

 the Origin and Growth of the Triradiate and 

 Quadriradiate Spicules in the Family Clath- 

 rinidse. 



By 



E. A. ITIiiicliin, M.A., 



Fellow of Merton College, Oxford. 



With Plates 38—42. 



Contents. page 



Preface ...... 470 



I. Introductory ..... 473 



(a) Methods of Investigation .... 474, 



(b) Anatomy and Histology of the Genus Clathrina . 477 



(c) System and Nomenclature .... 500 

 II. Descriptive — Observations upon the Development of the 



Spicules . . . . .502 



(a) The Development of the Triradiate Systems . . 504 

 {b) The Origin of the Fourth or Gastral Eay of the Quadri- 

 radiate Spicules . . . .517 

 (c) Some Observations on the Formation of the Monaxon 



Spicules ..... 523 



Appendix— (1) The Triradiates of Clathrina clathrus . 525 



(2) The Influence of the Nucleus on the Secretion 



of Lime .... 526 



(3) The Granules of the Dermal Layer . . 526 



III. Historical — Earlier Observations upon Calcareous and other 



Spicules ..... 528 



(a) Observations upon Spicule Formation . . 528 



(1) Observations upon Siliceous Sponges . . 529 



(2) Observations upon Calcareous Sponges . . 532 



(b) Observations upon the Structure and Composition of the 



Calcareous Spicules .... 542 



IV. Theoretical— On the Origin and Evolution of the Spicules of 



Calcareous Sponges .... 543 



Addenda. — A. On the Presence of an Axial Organic Filament in 



the Spicule Rays . . . 569 



B. On Compound Spicular Systems behaving as Single 



Crystals .... 572 



Bibliography . . . . .579 



Description of the Plates .... 582 



VOL. 40, PART 4. NEW BEE. L L 



