POHIFERA [sponges]. 23 



The body substance, which is permeated by the canal system, High Cases 

 contains in the present species minute needles of silex (Fig. 3d), m."i^"'"'n."jfg 

 each -gL of an inch in length, scattered rather irregularly throughout i and 2 a, b. 

 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 a])out 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. 



Hfdichondrla 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, Chdhrina hlanca (Fig. 4) and Case 2a, in its earliest 

 gtage 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 ciHatum (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 Syron nimm.yi (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, siHca, 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, CALCAREA. Calcareous Sponges, Skeleton calcareous 



