POKIFEUA (sponges). 49 



reproduction also, ami are eiipalile of producing at certain seasons eggs or ova, 

 and sperm-cells. 'llie impregnated ova dcveloj^ tliemselves into embryos, 

 which are provided with numerous cilia Ijy means of which they swim ahout 

 freely. Finally, upon linding a suitable locality, they fix themselves to some 

 solid oTiject, lose their ciiia, and grow up into the fixed adult. Indeeil, as a 

 general if not universal rule, the embryos of the sponges are [H'ovided with 

 cilia, and are thus capable of active locomotion In this way is secured the 

 extension over a wiile are.i of these otherwise fixed and plant-like organisms. 



As to tlie distribution of the Sponge.« in .space, the common 

 Spongillce and certain allied types are known as " fresh - water 

 sponges," as they occur in lakes and rivers ; but most sponges are 

 inhabitants of the sea. The common Horny Sponges are mostly 

 found attached to stones or other foreign olijects Ijetween tide- 

 marks, or in comparatively shalhjw water. C'ei'tain singular types 

 (Cliond) burrow into shells, in which they form winding and 

 branched tunnels, but tlie mechanism by which this is effected is 

 not clearly known. The Glass Sponges are all marine, and are 

 mostly found at comparatively great depths in the sea, being often 

 anchored to the mud or sand at the bottom by longer or shorter 

 root-fibres of flint. Lastly, the Calcareous Sponges are all marine, 

 and are mostly confined to shallow water, or live between tide- 

 marks. They are all small in point of size, and delicate in texture. 



