CHAPTER VII 



POEIFERA (sponges)^ 



INTEODUCTION — HISTORY DESCKIPTION OF HALICHONDR I A 



PA NICE A AS AN EXAMPLE OF BRITISH MARINE SPONGES 



AND OF EPHYDATIA FLUVIATILIS FROM FRESH WATER 



DEFINITION POSITION IN THE ANIMAL KINGDOM. 



Sponges occupy, perhaps, a more isolated position than any other 

 animal phylum. They are not only the lowest group of multi- 

 cellular animals, but they are destitute of multicellular relatives. 

 They are all aquatic and — with the exclusion of a few genera 

 found in fresh water — marine, inhabiting all depths from between 

 tide marks to the great abysses of the ocean. They depend for 

 their existence on a current of water which is caused to circulate 

 through their bodies by the activity of certain flagellated cells. 

 This current contains their food, it is their means of respiration, 

 and it carries away effete matters. Consequently sponges cannot 

 endure deprivation of oxygenated water except for short periods, 

 and only the hardiest inhabit regions where the supply is 

 intermittent, as between tide marks. This also renders useless 

 attempts to keep specimens in tanks, unless the water is 

 frequently renewed. 



The outward appearance of sponges has an exceptionally wide 

 range, so that it is difficult to give a novice any very definite 

 picture of what he is to expect when searching for these animals. 

 This diversity is in part due to the absence of organs of sufficient 

 size to determine the shape of the whole or limit its variation, 



' To Professor W. J. Sollas, Sc.D., F.R.S., who undertook to write the chapters 

 on Porifera when the work was first planned, the Author and the Editors are 

 indebted for his kind assistance in reading and criticising this article. 



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