104 THE SPONGES. 



gether with the tangential rays of the triaenes supporting the dermal 

 membrane. 



b. Larger oxeas of tlie same character as above, reaching 25 mm. in 

 length, project in rather small number from both surfaces. 



c. Rhaphid oxea, smooth, cylindrical, 3-5 mm. x 12 /a; not very abun- 

 dant; disposed without order in the interior, often but not always in 

 bundles of two or three. 



2. Triacnc. Rhabdome, except in rare cases, reduced to a rounded knob 

 (Fig. 13, Plate 13), the spicule becoming a triod ; exceptionally appear- 

 ing as an elongated pointed ray, shorter than, or about the length of, the 

 cladi. Cladi 1.0 mm. to 700 jx long, smooth, tapering to a point, and 

 curving slightly in the direction of the reduced ray. The three cladi are 

 usually but not always of about the same length, and are frequently slightly 

 twisted or deformed. 



The triaenes are very abundant just internal to the dermal membrane at 

 both surfaces ; here arranged in several layers (about three), with the cladi 

 tangential to the surface. The cladi of adjoining spicules overlap, and 

 together with the tangentially placed oxeas, they establish a hypodermal 



framework. The same triaene is also scattered sparsely through the 



interior along with the internal oxeas, becoming fairly abundant near the 

 margin of the sponge body. In the interior the cladi are not arranged 

 tangentially to the surface, but at various angles. 



Microscleres. 



3. Anniilated microxea, Fig. 12, Plate 13. Spicule, 400-500 ja x 8-16 /x, 

 curved, and tapering at each end. Surface covered with minute annular 

 ridges, which give the impression of being parts of a discontinuous spiral 

 ridge. At the ends, the ridges give place to a mere granulation of the 

 surface. Very abundant in the hypodermal region at both surfaces, and 

 here for the most part tangentially placed. Fairly abundant in the interior, 

 and here disposed without order. 



4- Microxea, Fig. 14, Plate 13. Spicule commonly about 120 /a 

 long, slightly curved, tapering at each end. Surface nearly but not 

 quite smooth, most minutely roughened. Abundant in the interior and 

 in the hypodermal region. Possibly only a young form of the annu- 

 lated oxea. 



5. Spimsters of dermal membrane. Fig. 2 a, Plate 14. Spicule length, 

 20 /A ; rays, 2-3 /x, long, numerous, blunt-pointed. Whole spicule smooth. 



