906 THE HOMOC(ELA DESCRIBED FROM AUSTRALIA, 



surface which divides the Entoderm from the Mesoderm, They 

 stand at equal angles, each ray measuring 0-04 — 0-05 x 0004 mm. 

 The fourth ray penetrates the Entoderm, and is situated radially. 

 Like the former it is conic, and ofteia slightly curved. The three 

 tangental rays are at their base convex towards the axis, the 

 radial ray concave towards the Osculum. The radial ray measures 

 0-02 — 0-04 X 0-0025 mm., and it is accordingly as long or shorter 

 than the others. 



2. The Parenchymal quadriradiate spicules are all parallel 

 three short and equal rays tangental and one longer radial 

 ray pointing outward. These s])icules vary very much in size ; 

 the radial ray is straight, slender, and conic, measuring 

 0-03— 0-05 X 0002— 0-003 mm. The tangental rays all have the 

 same angle of 120° between each other, and are slightly curved at 

 the ])ase, the convex side towards the axis. They measure 

 0-0075—0-01 X 0-002—0-003 mm. ; they are therefore much stouter 

 than the radial ray. 



These spicules are found throughout the Mesoderm, the larger 

 ones towards the inner surface. In no case do they penetrate 

 either the Entoderm or the Ectoderm. 



3. The Parenchymal tri-radiate spicules are situated below the 

 Ectoderm, and do not penetrate it. Their position is irregular, 

 their rays lie at equal angles, and appear much curved and 

 irregular, measuring 0-05 x 0-003 mm. These spicules are not nearly 

 so numerous as the former. 



4. The Dermal tri-radiate spicules are similar to the former in 

 shape and size ; the diflerence lies mainly in the fact that these 

 always possess perfectly straight rays, one of which is always 

 situated radially, and penetrates the Ectoderm. They are found 

 in rings around the tufts of acerate spicules near the summit of 

 the ciliated chamber, and also intermingled with the latter. 



5. The thick Dermal acerate spicules in tufts of 15 — 25 on the 

 summit of each ciliated chamber stand at an angle of 45 to the 

 axis of the Sponge, and point upward toward the Osculum ; they 

 are pointed at both ends and spindle-shaped. The thickest part is 

 nearer the proximal end, they measure O'O — 1 x 0-01 mm. 



