2 12 PORIFERA 



Vosmaer. Again, some Monaxonida approximate to Tetractinel- 

 lida, and we might, with Vosmaer, unite them under the title 

 Spiculispongiae. This proceeding, though it has the advantage 

 of being at least an attempt to secure a natural classification, 

 involves too much assumption when carried out in detail to be 

 wholly satisfactory. 



Sub-Class I. Tetractinellida.^ 



Tetractinellida appear to flourish best in moderate depths from 

 50 to 200 fathoms, but they are found to be fairly abundant also 

 in shallower water right up to the coast line, and in deep water 

 up to and beyond the 1000 fathom line. Occasionally they lie 

 free on the bottom, but are far more commonly attached ; fixation 

 may be direct or by means of rooting spicules ; the occurrence of 

 a stalk is rare. There is great variety in the root tuft, which 

 may be a long loose wisp of grapnel-headed spicules, as in many 

 species of Tetilla, or a massive tangle, as in Cinachyra harbata ; 

 in these cases the sponge is merely anchored, so that it rests 

 at the level of the surface of the ooze ; in other cases, e.g. 

 Thenea wyvillei, the root tuft consists of a number of pillars of 

 spicules which raise the sponge above the level of the ooze, into 

 which they descend and there become continuous with a large 

 dense and confused mass of spicules. The parachute-like base of 

 Tetilla casula invites comparison with the " Crinorhiza " forms of 

 some Monaxonids (p. 216). 



Two Orders are distinguished thus : — 



I. Choristida. — Tetractinellida witli qiiadriradiate spicules, wMch are 

 never articulated together into a rigid network. 

 II. LiTHisTiDA. — Tetractinellida with branching scleras (desmas), which may- 

 or may not be modified tetrad spicules, articulated together to form 

 a rigid network. Triaene spicules may or may not be present in 

 addition. 



Order I. Choristida. 



Plaldna monolopha, from the Adriatic and Mediterranean, 



famishes a connecting link between the Ehagon stage and other 



Tetractinellida. The choanosome is simply folded ; there is no 



distinct ectosome ; the chambers are eurypylous. The skeleton 



' Sollas, Challenger Monograph, xxv. 1888. 



