PORIFEBA (sponges). 31 
in one direction (one-rayed) or in two directions (two-rayed), 
but in the latter case the two rays lie along a single axis ; 
the axis is straight or curved. (2) Triaxons (Fig. 9), spicules 
that grow out from a single origin in both directions along 
three axes at right angles to one another, thus producing 
six rays, some of which may, however, be suppressed. 
(3) Tetraxons (Fig. 11), spicules that grow out in one 
direction only along four axes which meet at equal angles ; 
thus there are four rays, one or more of which may be 
suppressed. In the latter case tetraxon rays may be distin- 
guished from those of triaxons by meeting at an angle of 
about 120° instead of 90° or 180°. (4) Polyaxous (Fig. 8, a, d), 
which grow out along several axes radiating from a common 
centre. The modifications and associations of these four 
types can be gathered from the accompanying figures. These 
types are more conspicuous in the larger spicules that build 
the skeleton ; there are also smaller fiesh spicules or skin 
spicules of more irregular shape (Fig. 8, 
Frequently sponges are divided into only two (Classes : — 
(I.) Tliose with spicules of calcite, called Calcispongiae or 
Calcarea; (II.) all the rest, called Silicispongiae or Silicea. 
There is however considerable difference between those 
Silicea having six-rayed spicules or triaxons, and all the 
others, so that the following classification has been pro- 
posed. 
Branch : CALCISPONGIAE. 
Class I.-.- CALCAREA. 
•Skeleton of calcareous spicules, either monaxon or 
tetraxon or both (Fig. 8, h-n). 
Grade A. — Homocoela. Body a simple sac, which 
branches in the adult, but retains the simplest type of canal 
system, and is lined throughout by collar-cells. Although 
the most primitive forms, these have not yet been found 
fossil, probably because of their small size and imperfect 
skeleton. 
Grade B. — Heterocoela. Canal-system broken up into 
separate flagellated chambers, to which the collar-cells are 
restricted. According to the degree of complexity of these 
chambers, and according to the shapes of the spicule.s, the 
Heterocoela are divided into six families, of which only 
one, the Pharetronidae, is largely represented in the fossil 
state. The Sycettidae, in whicli the chambers are radially 
arranged round the central cavity, are doubtfully represented 
Gallery X. 
Wall-case 
8b. 
