PORIFERA [SPONGES]. 25 
Sub-Class II. Demospongi^ {demos, multitude), including all 
sponges other than Calcarea and Hexactinellida. 
Grade I. Tetractinellida ( tetra , four ; aktis, ray). Four-Ray 
or Anchor Sponges. Siliceous spicules typically with four axes and 
four rays ; also certain fleshy sponges (Carnosa) are included here. 
Grade II. Monaxonida {monos, single ; axon, axis). Monaxon 
Sponges. Siliceous spicules rod- or pin-shaped. 
Grade III. Keratosa {Jeer as, horn). Horny Sponges. Skeleton 
of horny fibre. 
Grade I V. Myxospongida {myxa, slime). Slime Sponges. With- 
out a skeleton. 
Class I. CALCAREA [Calcareous Sponges]. 
The Calcareous Sponges form a comparatively small group, only 
about 200 recent species being known. They live for the most part 
in shallow water, and prefer shady sheltered localities. 
The skeleton is composed of spicules of carbonate of lime, which 
are either separate, or, in a few instances, fused into a solid frame- 
work. The spicules are either three-rayed, four-rayed, needle-shaped, 
or, in one instance, spherulitic. 
The Asconidce, which are the simplest of all sponges, are formed 
of thin-walled branching tubes, lined with collar-cells throughout 
their inner surface. The tubular branches may be separate {Leuco- 
solenia), or may join to form a network {Glathrina). 
In the Syconidce the collar-cells are restricted to “ radial tubes ” 
surrounding a central cavity devoid of collar-cells. 
The beautiful Ciliated Sycon {Sycon ciliatum ) is one of the 
commonest sponges round our coasts, where it is found attached to 
rocks and seaweeds. This sponge, which is usually about an inch in 
height, has the form of a little oval white sac with a silvery crown of 
spicules round the orifice (oscule). The crown of spicules is fully 
expanded when currents are passing through, but otherwise it is 
closed. The wall of the sac is formed of horizontally arranged 
closely-packed tubes each opening by a comparatively wide aperture 
into the vertical central cavity, but ending blindly on the outer 
surface of the sponge. Currents pass through microscopic orifices 
in the walls of the radial tubes into the interior of those tubes and 
into the central cavity, and finally leave through the oscule. 
A very large specimen of a Sycon Sponge, over eight inches in 
length, from Poole Harbour, is exhibited in the Case. 
High Cases 
I.-VI. and 
Table Cases 
1 and 2 a, b, 
Table Case 2a. 
