10 CLASSIFICATION OF THE ANIMAL KINGDOM. 



Sub-ord. 5. Echinonemata. Axinella, Acanthella. 

 Sub-ord. 6. Holorhaphidota. Halickondria, Isodictya, 



Reniera, Hymeniacidon, Cliona, Geodia, Tethya, Spon- 



gilla. 



ORDER III. SILICEA (Silicispongise). 



Sub-ord. 1. Lithistidae. Discoderma, MacAndrewia-, Cor- 



cdlistes, * Siphonia, *Aulocopium. 

 Sub-ord. 2. Hexactinellidae. Euplectella, Holtenia (fig. 7), 



Hyalonema, Dactylocalyx, * Ventricidites. 



ORDER IV. CALCAREA (Calcispongise). Grantia,Leucosolenia. 



The most unnatural point in the above classification is the union under 

 the head of " Ceratose Sponges " of types like Euspongia, in which 

 spicules are not developed, with other types in which the horny skeleton 

 is accompanied by siliceous spicules, or may be even replaced by the 

 latter. A more natural classification probably is that adopted by Zittel, 

 in accordance with which the Sponges are divided into the following 

 orders : 



ORDER I. MYXOSPONGLE. Halisarca. 

 ORDER II. CERATOSPONGLE. Euspongia (Sponges of com- 

 merce). 



ORDER III. MONACTINELLID^E. Ralichondria. 

 ORDER IV. TETRACTINELLID^E. Geodia, Tethya. 

 ORDER V. LITHISTIDAE. Discoderma. 

 ORDER VI. HEXACTINELLID^E. Holtenia. 

 ORDER VII. CALCISPONGI^E. Grantia. 



The systematic position of the Sponges has been a matter of much 

 controversy among naturalists. Their animal nature is now universally 

 admitted, and there is also no substantial difference of opinion as to the 

 broad outlines of their anatomical structure. In all known forms of the 

 Sponges, the organism consists of an aggregate of protoplasmic bodies 

 (the " sponge-particles " or " sarcoids "), which differ in their characters 

 in different parts of the Sponge, or at different periods of its life, but 

 which are probably all fundamentally the same. Some of the sponge- 

 particles precisely and in every respect resemble Flagellate Infusoria, 



