58 INTRODUCTION TO ZOOLOGY chap. 



viduals are uuited together in an irregular flat colony by a 

 gelatinous substance, those individuals on the outside being 

 all typical collared flagellate forms, whilst those in the centre 

 are non- flagellate and have amoeboid movement. Such 

 Amoeboid cells are also found in the mesogloea of Sponges. 

 Sponges, however, are much more advanced and complex 

 than any Protozoa, and difier from them in the important 

 characteristic of their mode of development from an egg cell, 

 in which they approach the Coelenterates, though they are 

 too peculiar in other ways, as has been shown, to be classed 

 with them. 



Classification of the Sponges mentioned in Chapter IV. 



Class I. Calcarba. — Calcareous Sponges. 



Orantia compressa, the Purse Sponge. 

 Clathrina blanca, the White Lattice Sponge. 

 Sycon ciliatum, the Ciliated Sycon. 

 II. Hexactinbllida. — Six-rayed or Glass Sponges. 



Euplectella aspergillum, Venus' Flower-basket. 

 III. Demospongiae. — Common Sponges. 



Hippospongia equina, the Common Bath Sponge. 

 Spongia officinalis, the Turkey Cup-sponge. 

 Spongilla laeustris, the Pond Sponge. 

 Ephydatia {Spongilla) fluviatilis, the Eiver Sponge. 

 Halichondria panicea, the Bread-crumb Sponge. 

 Ghalina oculata. 



Practical Work on Sponges 



1. Obtain a few small specimens of a fresh-water sponge, and 

 place them in a vessel of water. Drop in some fine carmine 

 powder, in order to make visible the currents produced in the 

 water by the living sponge. 



Break one piece of sponge open and look for the little yellow 

 spherical gemmules ; keep them separate and watch their develop- 

 ment into new sponges. 



2. Collect sponges at the seaside ; they will be found attached 

 to rocks, shells, seaweeds, or submerged timber. Sketch any well- 

 defined forms, and identify by reference to Bowerbank's Monograph 

 of British Spongidae (4 vols.; vol. iii. gives pictures of many of the 

 commonest sponges). 



