COMMON GROUP. 



54i 



stony-sponges, are usually brittle and friable, or of stone-like hardness. The four- 

 rayed sponges belong mostly to shallow water, but a few specimens have been 

 obtained from depths of nearly two thousand fathoms. 



The Fleshy Sponges, — Order Carnosa. 



These form a small group of uncertain systematic position, their chief features 

 being the possession of a tough rind, enclosing a softer pith, the absence or slight 

 development of a skeleton, and the highly-developed canal-system. They appear to 

 be related to the four-rayed sponges. The soft pith contains the flagellated chambers 

 and the canals leading to and from them. The genus Chondrilla possesses isolated 

 siliceous spiny spheres, especially situated along the courses of the canals and 

 beneath the rind. The allied Chondrosia of the Mediterranean takes the form of 



sea-kidney leather sponge {Chondrosia reniformis). a, Specimen cut open. 



leathery knobs or cakes with a slimy surface. The usually solitary oscule is 

 irritable, and contracts slowly when the sponge is taken from the water. Fishermen 

 call this sponge, sea -flesh or sea -kidney. The ground -substance contains no 

 skeleton of silica or horny material ; and the in-current and out-current canals form 

 two sets of tree-like branched systems with the flagellated chambers interpolated 

 between the final twigs of each. 



Single -Rayed Sponges, — Order Monaxonida. 



These sponges are those most frequently met with on the British shores and 

 in shallow water throughout the world. The skeleton is mainly built up of 

 uniaxial siliceous needles or rods, which may be isolated and scattered, or united 

 into bundles by the horny cementing substance, spongin ; while the bundles may 

 be joined in various ways to form scaffoldings for the support of the soft parts. 

 The spicules are shaped like spindles. In addition to the large spicules forming 

 the bulk of the skeleton, and on this account called skeleton-spicules, in some 

 groups minute forms abound in the soft substance, and are termed flesh-spicules. 

 The latter are frequently shaped like buckles or double anchors, with prongs at 

 each end. A transition can apparently be traced from this group to the horny 



