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,—O rder 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 sroNGE [Chondrosia renifonnis). 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 tinal twigs of each. 
Single-Bayed 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 011 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 
