170 ZOOLOGY 



form of the skeleton too which gives the characteristic form to 

 the individuals and colonies of the different species. 



210. Nutrition. — The food of sponges is essentially similar 

 to that of the single-celled Protozoa. It is carried in by the 

 water currents, which enter the pores, pass along the canals 

 lined with the collared flagellate cells into the cloaca, and from 

 there reach the exterior by way of the osculum. The food par- 

 ticles are taken up principally by the gastral cells lining the 

 radial chambers and by the amoeboid cells which belong to 

 the mesenchyma. In these cells digestion takes place as in 

 Amceba. The indigestible parts of the food are returned to 

 the current and are eliminated through the osculum. There is 

 no other circulation. The digested food apparently diffuses 

 from cell to cell or is carried by the amoeboid cells. Respira- 

 tion occurs through all the cells which are in contact with 

 the water. 



211. Sensation and Motion. — Sponges are fixed and vege- 

 tative in their adult life, and show very little of the more 

 active functions. In addition to the ciliated and amoeboid cells 

 already described, the pores may be slowly closed in response to 

 stimulus. Contractile elements have been described as occur- 

 ring in the epithelium of these regions. It is uncertain whether 

 there are any nervous elements. The oscula open and close 

 under certain conditions, the flagellate cells work in unison, and 

 their rate may be caused to vary by change of conditions. 



212. Reproduction by outgrowth or budding is common. 

 In this way large colonies arise from a single individual. New 

 colonies may arise, especially in the fresh-water sponges, by 

 the separation of gemmules or groups of cells produced asexu- 

 ally within the mesenchyma. These, after a period of rest, 

 escape and produce new individuals. Sexual reproduction also 

 occurs in all sponges. The ova and sperm are developed in 

 the mesenchymatous layer. The male and female cells origi- 

 nate from the same individual (hermaphroditism). Usually 

 however the sexes mature at different times. 



