146 



SPONGES 



compromise between logical necessities and natural affinities. It is 

 hoped that the classification here adopted represents such a com- 

 promise in which the disturbance of the true relationships is reduced 

 to the unavoidable minimum. 



The following scheme represents the four main sub-classes and 

 their principal orders. By means of brackets placed on the right, 

 the (perhaps) more natural affinities of the sub-groups are indicated : 



CLASS DEMOSPONGIAE (Sou,) 



GRADE I. TETRAXONIDA (Ldf.) 



Order 1. Carnosa (Crtr.), Tops. 

 2. Tetractinellida (Marshall). 



GRADE II. MONAXONIDA (R. and D.) 



Order 3. Hadromerina (Tops.) 

 Sub-Order 1. Aciculina (Tops.) 

 Sub-Order 2. Clavulina (Vosm.) 



Order 4. Halichondrina (Vosm.) 

 GRADE III. KERATOSA. 



Order 5. Dictyoceratina. 

 ,, 6. Dendroceratina. 

 GRADE IV. MYXOSPONGIDA (Soil.) 



Family 1. Halisarcidae (O.S.) 

 2. Oscarellidae (Ldf). 



DETAILED CLASSIFICATION OF THE DEMOSPONGIAE. 



GRADE I. TETRAXONIDA. 

 Demospongiae typically witli tetraxon spicules. 



ORDER 1. Carnosa (Crtr.X Tops, emend. 



TetraxonidaVith the spicules greatly reduced in size, and even want 

 ing ; no diactinal megascleres or triaenes with long rhabdomes. 



SUB-ORDER 1. IMICROTRIAENOSA, Tops. 



The characteristic spicules are triaenes with short rhabdomes, not 

 specially differentiated in the ectosome or the choanosome, and often 

 variously ornamented or of aberrant types (amphitriaenes, mesotriaenes, 

 etc.) ; microscleres of various kinds. A heterogeneous collection of 

 sponges, of diverse affinities : " chainons de chaines brisdes, derives sans 

 intermediates connus " (Topsent). Not divided into families. Genera 



t Recent aud fossil. 



