340 THE VOYAGE OF H.M.S. CHALLENGER. 



structure of the shell. Here we restrict the definition to those spongy S p h se r e 1- 

 1 a r i a in which the central capsule and the enclosing spongy cortical shell are 

 ellipsoidal or cylindrical, therefore each transverse section is a circle, and each meridional 

 section an ellipse, as in all Prunoidea. 



The Cortical Shell in all Spongurida is composed of a delicate framework of 

 irregularly branched and interwoven siliceous threads ; commonly this spongy structure 

 is rather dense or compact, but sometimes also very loose. In the simplest form, 

 Spongellipsis, the spongy cortical shell contains a large cavity, in which lies freely the 

 central capsule. In Spongurus this cavity is completely distended by a spongy frame- 

 work. In this case the solid spongy shell becomes often prolonged, and its original 

 ellipsoidal form passes over into a cylindrical one (as in many Zygartida). Sometimes 

 (particularly in Spongocore, PL 48, fig. 6) the cylinder becomes three-jointed by two 

 more or less distinct annular constrictions. Spongocore is distinguished by an outer 

 veil, a thin lattice-lamella, which envelops the spongy shell and is connected with it 

 by radial beams. 



The Medullary Shell, absent in the Spongellipsida, is constant in the second sub- 

 family Spongodruppida. It is either a simple latticed shell (Spongodruppa) or double, 

 composed of two concentric latticed shells (Spongoliva] ; its form is either spherical or 

 ellipsoidal. It lies in the middle of the central capsule, and is connected by radial 

 beams (perforating the membrane of the latter) with the enveloping spongy cortical 

 shell. 



In many Spongurida the surface of the shell is armed with radial spines, and in 

 some genera (Spongoprunum, Spongatractus, &c., PL 17, fig. 12), on both poles of 

 the main axis, are developed two strong opposite polar spines, as in many other 

 Prunoidea. 



The Central Capsule of the Spongurida is either ellipsoidal or cylindrical. Only in 

 Spongellipsis it lies freely in the internal cavity of the spongy shell. In all other genera 

 it is perforated by a part of the skeleton ; in the Spongodruppida it contains the simple 

 or double medullary shell, and the radial beams which perforate its membrane and 

 connect the latter with the external spongy cortical shell. In Spongurus and the allied 

 genera (Spongocore, Spongoprunum) the whole central capsule is filled with a spongy 

 framework which also envelops its surface. 



