CLASSIFICATION OF THE SPOXGES. 325 



between the two, probably derived fix>m the epiblast. The embryo now lengthens into a cylindrical 

 form, pores appear in its sides, and an osculum opens at the free end, the primitive mouth having 

 become closed soon after the attachment of the larva (Fig. 1 3, G, H). 



The spicules of the Sponge always develop in the niesoderm, probably in all cases, certainly in 

 some, as the products of single cells. It is a curious fact that they appear in the embryo before it 

 becomes attached, sometimes even before it has left the body of its parent. 



Classification. The position of the Sponges in the organic world was long the subject of con- 

 troversy, and it was not till after the fundamental discoveries of Robert Grant, in 1825, that their 

 right to a place in the animal kingdom was universally admitted. After the difficulty which had 

 been experienced in making good their claims to an animal nature, it would naturally be expected 

 that they would be assigned but a very lowly place amongst their recognised associates, and accord- 

 ingly we find them originally relegated to the Protozoa, the lower of the two sub-kingdoms into which 

 the animal world is now divided, the other being known as the Metazoa. The embryological history 

 of the Sponge, and the sub-ordination of its individual cells to the unity of the complex whole which 

 they form, ensured for them, however, the highest place in the s\ib-kingdom. But they were not long 

 allowed to enjoy an ignoble repose. Already, in 1854, Leuckart advanced them into the higher sub- 

 kingdom as members of the Coelenterata. At that time, however, naturalists were not prepared to 

 acknowledge the justness of this promotion, and it was not till Haeckel, in a brilliant but too 

 imaginative work, came forward in its support, that it found any general acceptance. Previously, 

 however, in 1866, Professor James Clark had been led, by his discovery of the resemblance of the 

 collared celk of the Sponge to the flagellated infusoria, to regard the Sponges as mere aggregates of 

 these Protozoa, but this view, though earnestly supported by Carter and Saville Kent, is opposed to 

 the general opinion of most naturalists. The searching investigations into the structure and embry- 

 ology of the Sponges since the publication of Haeckel's views, leave little doubt as to their Metazoic 

 nature. But with regard to the nearness, or otherwise, of their relations to the Crelenterata, the 

 greatest doubt still exists. The writer originally regarded them as Coelenterata, which differ from all 

 other members of the class, in the fact that their embryos attach themselves by their oral instead of 

 their aboral extremity, but in spite of certain remarkable resemblances of the larvae to those of the 

 Crelenterata the balance of evidence seems in favour of those who, with Balfour, regai'd the Sponges 

 as forming a separate class, quite independent of the Coelenterata, and situated at the very bottom of 

 the Metazoic sub-kingdom. 



For the subdivision of the Sponges into smaller groups we are chiefly indebted to Oscar Schmidt 

 and Carter, not to forget Haeckel. The accompanying table is founded chiefly on the classifications 

 proposed by them, with modifications, which may possibly be found convenient. 



CLASS SPONGI^E. 



Orders. Sub-orders. Families. 



MYXOSPONGIJE ( Halisarcidae 



( Chondrosiadse. 



( Ascones. 

 CALCISPONGI^: , . . . J Leucones. 



( Sycones. 



/'Kenierinae. 



.- | Suberitidinae. 



[ Monaxonidse * * <, { Desmacidinse. 



] Echinonemata. 



I Chalinidaa. 



Tetractinellidsa 



... 



| 



Hexactinellid33 . : 



( JJictyomna. 



Ceratiiiidae. 



-n . 



JL sammonemata . 



MYXOSPONGUE. 



The Halisarcidic* characterised by the entire absence of any skeletal parts, are represented by 

 the single genus Ilalisarca, comprising a number of small smooth soft Sponges, which grow iu 



* Greek, hah, halos, the sea, sarx, sarcos, flesh. 



