PORIFERA [SPONGES]. 23 
The body substance, which is permeated by the canal system, 
contains in the present species minute needles of silex (Fig. 3d), 
each -gV of an inch in length, scattered rather irregularly throughout 
the body tissues, but sometimes forming an obscure scaffolding. In 
the skin, the needles are joined into bundles, which unite at 
their ends to form a network, in the meshes of which are groups of 
pores. The body-tissues are composed of cells of various kinds, some 
Of which are concerned in nutrition, others in secreting the skeleton ; 
others, again, line the surface of the canals and of the body. 
At certain seasons the body develops egg-cells, which, after 
fertilisation, form little oval ciliated embryos ; these swim about for 
a day or two, settle down, and become sponges, the ciliated cells 
becoming collar-cells. The organism, being unable to roam in 
search of food, sets up currents which convey food to it. 
Halichondria is a Siliceous Sponge belonging to the order Mon- 
axonida, because its skeleton is composed solely of siliceous spicules 
having one axis. 
A very brief account of one of the simplest sponges may help 
further to elucidate the structure of these organisms. The Cal- 
careous Sponge, Glathrina Uanca (Fig. 4) and Case 2a, in its earliest 
stage forms a minute thin-walled sac opening at the summit by the 
oscule. The interior of the sac is lined with collar-cells, and the 
wall is perforated by tine pores. Currents enter through the pores 
and leave by the oscule. The thin wall is supported by three-rayed 
spicules of carbonate of lime. The canal system is here in its 
simplest form. In Sycon ciliatum (Table Case 2a) the wall of 
the sac gives off horizontally arranged tubular pockets, which alone 
are lined with collar-cells. A piece of the inner wall of the large 
specimen of Sycon ramsayi (2a) shows the honeycomb-like openings 
of the tubes. 
Classification. 
The composition and structure of the skeleton afford the most 
reliable characters for the classification of Sponges. The skeleton is 
composed either of calcium carbonate, silica, or horny material 
usually in the form of fibre. The calcium carbonate and silica are, 
for the most part, secreted in the form of spicules, which may be 
separate or fused together. A few sponges do not form a skeleton, 
A simple scheme of classification is given below : — 
Class I. CALC ARE A. Calcareous Sponges. Skeleton calcareous. 
High Cases 
I.-VI. and 
Table Cases 
1 and 2 a, b. 
