39 



{By W. Johnson Sollas, LL.D., 



THE great advance whiclt has been made during the 

 past fifteen years in our knowledge of the sponges 

 is due partly to the vivifying influence of the evolutional 

 hypothesis, but still more to the opportunities afforded by 

 novel methods of technique. To the strength and weak- 

 ness of the deductive method Haeckel's work on the Kalh- 

 sehwamme (6) i is a standing testimony, while the slow but 

 sure progress which accompanies the scientific method is 

 equally illustrated by the works of Schulze {20), who by 

 a masterly application of the new processes has more 

 than any one else reconstructed on a sure basis the general 

 morphology of the sponges. In the general progress the 

 fossil sponges have been involved, and the application of 

 Nicol's method of studying fossil organisms in thin slices 

 has led, in the hands of Zittel and others (24, jjr), to a 

 complete overthrow of those older classifications which 

 relegated every obscure petrifaction to the fossil sponges, 

 and consigned them all to orders no longer existing. 

 But, whilst many problems have been solved, still more 

 have been suggested. An almost endless diversity in 

 details differentiates the sponges into a vast number 

 of specific forms ; the exclusive possession in common of 

 a few simple characters closely unites them into a compact 

 group, sharply marked off from the rest of the animal 

 kingdom.^ 



^ These italic numbers refer to the bibliography which will be 

 found at page 54. 



^ Since this was written, in 1887, four large monographs, includ- 

 ing considerably over 2000 pages of letterpress, have been published on 

 the Sponges. Three of these, viz. : — Schulze on the Hexactinellida, 

 Ridley and Dendy on the Monaxonida, and Sollas on the Tetractinel- 

 lida appear as Reports of the " Challenger " Expedition, the fourth by 

 Von Lendenfeld on the " Horny Sponges " as a special volume issued 

 by the Royal Society. With this addition to our knowledge a longer 

 preface than this would be possible, but for the general student the 

 following amended classification of the Monaxonida will probably be 

 found sufficient. 



Order. — Honazonida. 



Sab-order 1. ASEMOPHORA, Sollas. 



Family 1. Homoeaphid^, Ridley and Dendy. — Megascleres either 

 oxeas or strongyles. No microscleres. Ex. : Halichondria. 



Sub-order 2. MENISCOPHORA, Sollas. 



The microscleres when present are sigmaspires, sigmas, or cymbas. 



Family 1. Heteboeaphid^, Ridley and Dendy. — Megascleres of 

 various forms, microscleres never cymbas. Ex. ; Rhizochalina, 0. S. 



Family 2. Desmacidonid^, 0. S. — Megascleres usually monactinal, 

 microscleres cymbas. Ex. : Desmacidon, 0. S. 



Sub-order 3. SPINTHAROPHORA, Sollas. 



The miorosclere when present is some form of aster. 



Group 1. HoMOSCLEEA. — The spicules are all microscleres. 



Family 1. AsTEOPEPLmai. — The microscleres are microxeas and 

 asters. Ex. : Astropeplus, Soil. 



Group 2. Heteeoscleea, Soil. — Megascleres are always present, 

 and sometimes microscleres. 



Demus 1. Centeospinthaea, Soil. — The microsclere when present 

 is a euaster. 



Family 1. AxiNELLlDiB, O.S. — Non-corticate, mesoderm coUen- 

 chymatous, chamber system eurypylous. The skeleton consists of 

 axial and radial spicular fibres. Ex. : Axinella,, 0. S. 



Family 2. DoETPLEBiDyE, Soil. — Non-corticate, mesoderm collen- 



SPONGES 



F.B.S., Professor of Geology, Triniiy College, Dublin.) 



Structure and Form. 



Description of a Simple Sponge. — As an example of Simple 

 one of the simplest known sponges we select Ascetta, sponge. 

 primordialis (fig. 1), Haeckel. This is a hollow vase-like 

 sac closed at the lower end, by which it is attached, 

 opening above by a comparatively large aperture, the 

 osculum or vent, and at the sides by numerous smaller 

 apertures or pores, which perforate the walls. Except for 

 the absence of tentacles and the presence of pores it offers 

 a general resemblance to some simple form of Hydrozoon. 

 Histologically, however, it presents considerable dif- 

 ferences, since, in addition to an endoderm and an 



Fig. 1.- 



-Ascetta primordialis^ Haeckel. 

 After Haeckel. 



ectoderm, a third or mesodermic layer contributes to 

 the structure of the walls ; and the endoderm consists of 

 cells (see fig. 21,^') each of which resembles in all essential 

 features those complicated unicellular organisms known 

 as choanoflagellate Infusoria (see Protozoa, vol. xix. p. 

 858). With this positive character is associated a nega- 

 tive one : nematocysts are entirely absent. The activity 



chymatous. Skeleton consisting of oxeas arranged without order. 

 Ex. : Dorypleres, Soil. 



Family 3. Tethyidj!, Vosm. — Corticate. Skeleton consisting of 

 radially arranged oxeas. The microsclere is a spheraster. Ex.: 

 Tethya, Lam. 



Demus 2. SpieaspinthaeA, Soil. — The microsclere is a spiraster. 



Family 1. SooLOPlDiB, Soil. — The cortex is thin and fibrous, with 

 radially arranged closely-packed microxeas and oxeas. The skeleton 

 consists of oxeas collected into radially disposed fibres. The micro- 

 sclere when present is an amphiaster. Ex. : Soolopus, Soil. 



Family 2. SnBEEiTir^, O.S. — Cortex with a skeleton of radially 

 arranged stjjes. Microscleres usually absent. The megascleres are 

 tylostyles. Ex.: Suberites, Nardo. 



Family 3. Spieasteellid.e, Ridley and Dendy. — The megascleres 

 are rhabdi or styles. The microscleres are spirasters or discasters. 

 Ex. : Spirastrella, 0. S. 



