PORIFEKA [sponges]. 25 



Sub-Class II. Dkmosi'oxcm; {<h'iiiii<, imiltkude), inchuliii^ all )(■ -^ ' -m-s 

 sponges other than Calcarea and llfxaftinellhla. ' ' 



Grade I. Tetractinkllida (/^-^v/, fdur ; a/rti.s,n\\). F«jnr-Uav , 

 or Anchor 8pon,ii:es. Siliceous siticules ty[)ically with four axes and 

 four rays ; also certain fleshy sponges (("arnosa) are includt-d here. 



Grade II. Moxaxonida (wo/?o.<t, single ; (i.roii, axis). Monaxon 

 Sponges. Siliceous spicules rod- or pin-shaped. 



Grade III. Keratosa (/f^m.s, horn). Horny Sponges. Skeleton 

 of horny fibre. 



Grade IV. Myxospongida (»/y.m, slime). Slime Sponges. With- 

 out a skeleton. 



Class I. ("ALCAREA [Calcareous Sponges]. 



The Calcareous Sponges form a comparatively small group, only TabloCa- 

 about 2ii() recent species being known. They live for the must ^wrt 

 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. necdle-shapL-d, 

 or, in one instance, spherulitic. 



The Asconidcc, which are the simplest of all spunges, are formed 

 of thill-walled branching tubes, lined with collar-cells throughout 

 their inner surface. The tubular branches may be separate {Leuro- 

 solenia), or may join to form a network {C'l/'thn'na). 



In the Syconidir tlic collar-cells are restricted to "radial tubes" 

 surrounding a central cavity devoid of collar-cells. 



The beautiful Ciliated Sycon {S'l/ron ci/iafin/i) is one of the 

 commonest sponges round our coasts, where it is found attached tu 

 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 (»f 

 spicules round the orifice (uscule). The crown of spicules is fully 

 expanded when currents are passing through, but otherwise it is 

 closed. The wall of the sai- is formetl of horizontally ariimgeil 

 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 i>ass through microscopic orifices 

 in the walls of the radial tubes into the inU-rior of those tul>es and 

 into the central cavity, and finally leave through the oscnle. 



A very large specimen of a Sycon Sponge, over eight inches in 

 length, from Poole Ilaibour. is exhibited in the Case. 



