PORIFEEA (sponges). 



31 



in one direction (one-rayed) or in two directions (two-rayed), Gallery X. 

 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) Polyaxons (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 flesh spicules or skin 

 spicules of more irregular shape (Fig. 8, h-g). 



Frequently sponges are divided into only two Classes : — 

 (1.) Those 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. The canal-system is 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 

 spicules, they are divided into six families, of which only 

 one, the Pharetronidae, is largely represented in the fossil 

 state. The Sycettidae, in which the chambers are radially wall-ease 

 arranged round the central cavity, are doubtfully represented 8b. 



