roRiFERA [sponges]. 23 



The boily substiuici', which is iK-riiK-atc'tl l»y tlie aitjal Ryst+rui, H 

 coiitciins iu the preseut species iiiiiiute needles nf silcx ( V'n:. Alt), } 

 each ^V of an inch in length, scattered rather irregularly throughout i 

 I he body tissues, ]>ut sonietiuies forming an oljscure scaflfcjlding. In 

 the skin, tlic needles are joined into bundles, which unite ai 

 their ends to form a network, in the meshes of whicii are gmups of 

 pores. The body-tissues are compcjsed of cells of various kinds, some 

 of which are concerned iu nutrition, others in secreting the skeleton ; 

 others, again, line the surface of the canals and of the b(Mly. 



At certain seasons the body develops egg-cells, which, after 

 fertilisation, form little oval ciliated embryos ; these swim about f(»r 

 a day or two, settle down, and become s{iongL'S, the ciliated cells 

 becoming collar-cells. The organism, being unaljle to roam in 

 search of food, sets up currents which convey food to it. 



HaJkhondria 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, (Jlathrina blanca (Fig. 4) and Case 2a, in its earliest 

 stage forms a minute thin-walled sac opening at the summit by tlic 

 oscule. The interior of the sac is lined with collar-cells, and the 

 wall is perforated by fine pores. Currents enter through the jtorcs 

 and leave by the oscule. The thin wall is supjtorted by three-rayed 

 spicules of carbonate of lime. The canal system is here in its 

 simplest form. In Sijcon ciliaftim (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 tSyco/i rammyi (2a) shows the honeycomb-like openings 

 of the tubes. 



Classification. 



The composition and structure of the skeleton alb>rd 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 hbre. The calcium carbonate and silica arc. 

 for the most part, seci-eted in the form of simculks. whii-h may Ik* 

 separate or fused together. A few sponges do not form a skeleton, 

 A simple scheme of classifi('ation is given Itelow : — 



Class 1. ('AL('.\l;lv\. Calcareous Sponges. Skeleton cal'ureons. 



