THE STBUCTURE AND LIFE-HISTORY OF SPONGES. 237 



conclusion. That nourishment can be taken up by indifferent ecto- 

 derm cells is shown by the tapeworm, which may be here used in 

 illustration on account of its highly-developed nephridia. 



The most probable explanation of the functions of the 

 epithelia of sponges seems to me to be the following : — A constant 

 current of water passes through the sponge. At the entrance to 

 the canal system there are numerous inhalent pores, which are 

 covered by very fine sieves ; at the opposite end a few large 

 oscula, through which the water is expelled, are observed. If 

 the sponge fed on solid substances floating in the water the 

 current would of course enter by the wide open oscula, and would 

 be ejected through the small pores. Many experiments have 

 shown that the water invariably enters through the small pores 

 and passes out through the oscula. At the entrance to the 

 ciliated chambers there is a further filtering arrangement, and the 

 water current, caused by the movements of the cilia in the whole 

 canal system, is here impeded. The water in the inhalent canals, 

 outside the ciliated chambers, is consequently under slightly 

 higher pressure and enters the canal-epithelium; here the sub- 

 stances necessary for the life of the sponge are taken up. Behind 

 the pores which lead into the ciliated chambers — that is to say, 

 in the chambers themselves — there is a lower pressure, in conse- 

 quence of the increase in width of the canals towards the osculum. 

 This facilitates the secretory function of the collared cells. As the 

 sponge does not take up any solid substances there are no faeces, 

 and the expulsion of useless substances devolves on the ciliated 

 chambers. The epithelium of the inhalent canals is probably 

 also respiratory in function. In this case the inhalent canals 

 would represent digestive and respiratory organs, and the ciliated 

 chambers nephridia. 



As the process of nourishment is cari'ied on endosmotically, 

 a great quantity of useless material is probably absorbed together 

 with the nutrient substances, and all this, together with the 

 whole of the material oxidised in the sponge, must be extracted 

 and expelled by the cells of the ciliated chambers. This may 

 account for their high development in sponges, just as it ac- 

 counts for the high development of nephi'idia in the tapeworm. 



Sponges are distinguished from other Coelenterates by their 

 highly developed mesoderm, and the degree of differentiation 

 attained by its elements. Whilst all the organs of the Hydro- 



