54° 
SPONGES. 
sponges the spicules typically possess four axes and four rays, and resemble 
caltrops. The fleshy sponges, with little or no skeletal structure, form a second 
group ; while a third group includes the monaxonid or uniaxial sponges, with the 
skeleton typically built up of needle-shaped spicules, with one axis. The horny 
sponges form a fourth group. 
Four-Rayed Sponges, —Order Tetractinellida. 
In this order the siliceous spicules of the skeleton have four axes and four 
rays, and typically are shaped like caltrops. The typical form undergoes numerous 
modifications, one of the commonest consisting 
in the lengthening of the vertical ray, and the 
bending of the other three rays towards the 
long shaft, an elegant anchor -shaped spicule 
resulting. The anchor form may, however, have 
originated from the branching of a uniaxial or 
rod-shaped spicule, and not from the alteration 
of a four-rayed caltrops form. The three prongs 
of an anchor may point downwards, upwards, or 
horizontally outwards, and in the last case they 
are frequently forked. The Lithistida (stony 
sponges), one of the groups into which the order 
is divided, are characterised by the presence of 
peculiar spicules, known as desmas, in which a 
minute rod or caltrops is surrounded by con¬ 
centric layers of silica; at the margin of the 
plate or disc thus formed, branched and often 
tuberculated processes are given off, which usually 
join or interlock with those of other spicules to 
form a dense stony skeleton; but sometimes the 
desmas are not linked together, and the lithistid 
sponge is quite soft. In addition to the larger 
forms,of spicules, such as anchors, which form 
the skeleton, there are minute, coiled, spiral, or 
stellate spicules scattered in the flesh. A well- 
developed crust is frequently present; and in 
Geoclia the crust is composed of solid siliceous 
globules packed into a layer, beneath which lie 
the anchors with the prongs next the crust, and 
the long pointed shafts passing in centripetally. 
The four - rayed sponges are divided into the 
groups Choristida and Lithistida. In the former 
the spicules are loose and separate; and in the 
latter desma spicules are present, and usually 
fused or interlocked so as to form a stony skeleton. The Choristida frequently form 
yellowish white, leathery, nodulated cakes, plates, and crusts.. The Lithistida, or 
SILICEOUS SPICULES OF FOUR-RAYED ANCHOR 
sponges (magnified 200 diameters). 
