76 THE OCEAN WORLD. 



species, and sometimes varying even in the same species. Some of 

 these resemble needles, others are pin-like, and others again resemble 

 very small stars. 



The physiological function of those tubes and orifices which present 

 themselves on all parts of the sponge has been interpreted in various 

 ways. Ellis, writing in 1765, supposes that they were the orifices of 

 the cells occupied by the polypi. In 1816, Lamarck still advocated 

 this opinion; and even now we find the observer, whose notes 

 M. Fre"dol has edited with so much judgment, asserting that " the 

 inhabitants of the sponge are a species of fleeting, transparent, gela- 

 tinous tube, susceptible of extension and contraction ; young polypes, 

 as we may call them, without consistence, without cilia; incipient 

 polypes, in short, of very simple but sufficient organization. The 

 animalcule of the sponge is a stomach, without arms, very simple, 

 very elementary in short, an animal all stomach !" 



This mode of considering the sponge is not conformable to the 

 views of the leaders of modern science, however. Mr. Milne Edwards, 

 for instance, in place of seeing in the sponge a collection of united 

 beings, forming as it were a colony, considers each to be an isolated 

 being, an unique individual. The innumerable canals by which the 

 sponge is traversed, according to that author, are at once the digestive 

 organs and breathing pores of the zoophyte. The vibratile cilia are 

 necessary to the renewed aeration of the water required as a respiratory 

 fluid in the interior canals of the sponge. The currents in these 

 channels have one constant direction. The water penetrates the 

 sponge by numerous orifices of minute dimensions and irregular dis- 

 position \ it traverses channels in the body of the zoophyte, which 

 reunite somewhat like the root of a plant, in order to constitute the 

 trunk and increase its substance ; finally, the water makes its escape 

 by special openings. According to this view, the channels of the sponge 

 have a kind of cumulative physiology, performing the two functions of 

 digestion and respiration. The rapid currents of aerated water which 

 traverse them lead into them the substances necessary to the nourish- 

 ment of these strange creatures, rejecting all excremental matter. At 

 the same time, the walls of these canals present a large absorbing 

 surface which separates the oxygen with which the water is charged, 

 and disengages the carbonic acid which results from respiration. 



Sponges contain true eggs, from which embryo polyps are 



