THE SPOXGIDA OR PORIFERA. 351 



We have already alluded to the Sack Spo?ige—Sycandra, 

 {Grantia) compressa (Fig. 4, Plate XIV.). The pores of this 

 sponge open into radial tubes or chambers, which, passing through 

 the thickness of the wall of the sponge, open into the central 

 cavity of the latter. One of these radial tubes, lined with its 

 ciliated or flagellate collared cells, is represented in the accompany- 

 ing diagram (Fig. 2, Plate XVI.) ; the arrows indicate the current 

 passing into the tube through the pore {a), the water being swept 

 along by the flagellate collared cells {c), through the efl"erent open- 

 ing {b) into the main cavity or cloaca of the sponge, whence it is 

 ejected from the large osculum (Fig. 4, a, Plate XIV.). Some 

 spicules {d, c) are shown in the diagram (Fig. 2, Plate XVI.), but 

 we are concerned chiefly with the flagellate and collared cells. 

 The general plan of this sponge, it will be noticed, is very simple ; 

 and the same remark applies to the Crowned Sponge — Sycatidra 

 {G?'antia) ciliata (Fig. 5, Plate XIV.). Our specimen shows a 

 group of four ; these are elongated, from one to one and a half 

 inches in length, cylindrical in contour, from about one-quarter of 

 an inch to one-third of an inch in diameter at the thickest part, 

 very slightly stalked, and very gradually increasing in size towards 

 the free end, in which is situated the single osculum. Each there- 

 fore is slightly club-shaped, and all are gently curved. The colour 

 is a pearly or greyish white, the entire surface bristly or hispid, and 

 so porous as to be partially transparent when held up to the light. 

 The terminal osculum {a) is surrounded by a crown of delicate, 

 brightly-glistening spiculse about one-tenth of an inch or more in 

 length ; and on the specimens being held up to the light and turned 

 slowly round, the general bristly appearance of the sponge is clearly 

 due to similar, though less lengthy spicule, which surround the 

 entrances to the innumerable pores. These spiculae are beautiful 

 examples of defensive spicidce ; they encircle every pore, and, 

 during gentle circulatory action, the fringe of each little circle falls 

 in so as to present a pointed cone of bristles against dangerous 

 intruders, whereas, when the currents are vigorous and full, the 

 cones of spiculae assume, by mechanical means, a cylindrical form. 

 The same description applies to the spiculae surrounding the 

 osculum (a), except that the bristly circle becomes of a more 

 radiate form as the water-current increases in force. 



