THE SPONGES. 105 



Spiral axis shows in projection two or three concavities on the same side. 

 Very abundant in the dermal membrane of both surfaces. 



6. Metasters of parenchyma, Fig. 2 c, Plate 14, Spicule length, 20 /a ; 

 rays, 7 fi, tapering to points, pretty closely set along the curved axis. 

 Whole spicule smooth. Abundant throughout the parenchyma. Spicules 

 pass through transitional forms into the very similar spirasters of the 

 parenchyma. 



7. Spirasters of parenchyma, Figs. 2 ^, 2 c?, Plate 14. Spicule length, 

 24-26 /a; ray length, 8 /x. Spicule resembles the metaster, with which it 

 intergrades, but both axis and rays are longer, and the rays less closely 

 set along the axis. Abundant in the parenchyma. 



Poecillastra cribraria, sp. nov. 



Plate 14, Figs. 9-12 ; Plate 15, Figs. 1-4 ; Plate 16, Figs 1, 3. 



Diagnosis. Body plate-like. One surface slightly convex and bearing the pores. 

 Opposite surface slightly concave and bearing the oscula. Main afferent and efferent 

 canals similar ; numerous and small ; radial to the corresponding surface. Pores thickly 

 and uniformly scattered; appearing to the eye to be localized in areas, Oscula, 70-200 /* 

 in diameter, occurring singly or in small groups as perforations of the membrane roofing 

 in a main canal. Megascleres. Oxea. Triaene, ectosomal, and with a rhabdome shorter 

 than the cladi. Microscleres. Microxea, 180 /a long, surface minutely roughened. Spi- 

 raster, dermal ; spicule length, 16 fi ; rays, 2-3 /x long. Plesiaster and plesiaster-metaster, 

 parenchymal ; spicule length, 24 [x ; rays, 12 /x long. 



Station 3405, 1 specimen. 



The specimen. Fig. 3, Plate 16, is a fragment from the peripheral 

 part of a plate-like sponge. The pore surface is slightly convex, and is 

 evenly rounded off at the free margin of the sponge. The oscular surface 

 is slightly concave, and meets the free margin along a sharp lin •. The 

 two nearly straight edges in the specimen are broken edges, the curved 

 edge representing the natural margin of the sponge. The fragment 

 measures 40 x 25 mm., and is 5 to 9 mm. thick. The color is light brown. 

 The sponge is firm but not hard, with both surfaces smooth though not 

 strictly even. 



The main afferent and efferent canals are very similar. They pass 

 radially into the body from the corresponding surfaces, are 1.5 to 2.0 mm. 

 apart, and have a diameter of 600 /a to 1,0 mm. Beneath the dermal 

 membrane on both surfaces, between the larger canals, are everywhere 

 abundant smaller spaces. 



14 



