METAZOA PORIFERA. 79 



into the sponge, and its lightness keeps it from being 

 buried (Hyatt, Stand. Nat. Hist, I, 1885, p. 66). The 

 Suberitidae offer fine examples of spiral and radiate 

 structure of the skeleton. This is seen in Stylocordyla 

 stipitata var. globosa (PL 78, figs. 1-3). Fig. i shows 

 spiral arrangement of the bands of spicules in one speci- 

 men ; fig. 2 is a longitudinal section of another speci- 

 men, showing radiate structure; and fig. 3 is a cross 

 section of the same, showing the longitudinal spicules of 

 the stem by which the sponge is attached and the radiate 

 arrangement of the spicules of the body part. The ends 

 of the spicules of the stem which have been cut are seen 

 near the center of the drawing. In most of the Monaxo- 

 nia there is more or less horny cementing material called 

 spongin. It is interesting to note that the chemical 

 composition of this substance is similar to that of 

 chitin, Krukenberg having given it the chemical for- 

 mula C 30 H 46 N 9 O 13 , while that of chitin is C 15 H 26 N 2 O 10 . 



Another member of the Suberitidae is Raphiophora 

 patera Gray (No. 79), which is on the top of Section i. 

 This is one of the largest species of the Porifera and its 

 size and shape have given it the name of Neptune's Cup. 



Cliona (No. 80) is a borer into the living and dead 

 shells of mollusks, especially the oyster, and into lime- 

 stone, etc. PI. 81, fig. i, represents the openings of the 

 young Cliona enlarged, and fig. 2 shows the work of the 

 sponge in the interior of the shell. Just beneath the 

 outer surface is a series of excavations, and narrow 

 passages connect these with another series of cavities 

 below. When the shell is completely mined, the sponge 

 swells out in a bulbous mass on the outside (fig. 3). 

 Having destroyed the shell, it will takesnnd into its body, 

 as seen in fig. 4, which is a section of the sponge show- 

 ing fine black sand in the tubes and cavities of the inte- 

 rior. It also surrounds stones and takes them in, as seen 

 in fig. 5. 



No. 82 is a specimen of Italian marble bored by 



