CALCAREOUS GROUP. 
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an angle of 120° with each other. The spicules are transparent and glassy when 
viewed separately, but white and opaque in mass. On placing a calcareous sponge 
in acid, the skeleton dissolves away with effervescence. The class is divided 
into two groups, namely, the Homocoela and the Heterocoela, in the former of 
which the collar-cells line the whole of the interior of the simple or branched 
sac; while in the latter they are confined to the radial tubes, or the ends 
of branched radial tubes or canals, the gastric cavity with a part of the 
canal system being lined with flat cells. The simple ascon sponge (Ascetta 
'primordialis ) forms a minute stalked sac open above, and with its thin walls 
perforated by pores and supported by triradiate spicules; the whole interior being 
lined with collar-cells. The simple sac may give off a stolon, whence arise other 
sacs, or it may branch, and the branches again divide forming a tree-like growth; 
or, lastly, the branches may join together and form a complicated mesliwork of 
sacs or tubes. The allied Leucosolenia, shown in the illustration, is a branched 
ascon found on seaweeds in the form of clusters of small white tubes. 
The ciliated sycon ( Sycandra ) and Grantia, are common among seaweeds and 
in rock-crevices on the British coasts; both sponges are sycons, i.e. with tubes 
radiating out horizontally from a central cavity, and in 
both are present the three kinds of calcareous spicules 
(needles, three-rayed, and four-rayed forms). The walls 
of the ciliated sycon are made up of closely-packed 
tubes, lined with collar-cells and opening into the vertical 
gastric cavity. Rows of three-rayed spicules, arranged 
in regular series, support the walls of the tubes, the 
blind ends of which are protected by tufts of needles; 
a layer of four-rayed spicules lines the walls of the 
central cavity, the fourth rays projecting inwards 
and upwards into the cavity so as to form a wall 
of spikes. When the current is flowing from the 
interior the crown of needle bristles round the oscule 
is expanded, but when the sponge is inactive the 
bristles fall tog-ether and cover the oscule. Grantia, lives on seaweeds or 
hanging down from rocks, and several specimens are usually found together. 
They resemble small white leaves, or flat bags, averaging about an inch in 
length and one and a half in width, though sometimes much larger. When 
quite young and small they possess only one oscule, but larger specimens possess 
several on the thin margins. When the sponge is active, the flat leaf fills out like 
a small paper-bag. The lexicons usually possess tubular or hollow knob-shaped 
bodies with thick walls, in which ramify a double system of canals, in-current 
from the surface to the flagellated chambers, and out-current from the latter to the 
gastric cavity. 
On making a section or teasing a fragment of a calcareous sponge in the 
spring, the minute embryos will often be seen. When the embryo leaves the 
parent sponge, it consists of an extremely minute oval cyst or vesicle ( v ) with a 
small central cavity, and is formed of two kinds of cells. The anterior half, or the 
part in front when the embryo is swimming, is composed of a number of long 
A CALCAREOUS LEUCON SPONGE, 
Leucandra (nat. size). 
