PRIMITIVE MULTICELLULAR ANIMALS 



and are ingested by the choanocytes, which are also responsible for main- 

 taining the water currents. In view of the rapid growth of many sponges, the 

 nutrition is evidently very effective. The metabolism of sponges, which be- 

 cause of the structure of these animals is difficult to study, is assumed to be 

 similar to that of other animals in which it is better understood. 



When a sponge is exposed to a suspension of carbon particles (India ink) or 

 carmine granules in sea water, some of the particles enter the surface open- 

 ings with the currents of water. Later, when bits of the sponge are examined, 

 the granules are found within certain of the cells, just as such particles are 

 seen after ingestion by a paramecium (Fig. 9.1 1). Unicellular organisms have 

 been observed undergoing ingestion in a similar manner in sponges. Diges- 

 tion is undoubtedly intracellular, as in Protozoa. In the calcareous sponge 

 Scypha, the food particles are ingested chiefly by the choanocytes and are 

 then passed into nearby amoebocytes. Since the amoebocytes are migratory 

 and are capable of differentiating into other cell types, the food may thus be 

 distributed to all parts of the sponge. Storage of reserves, carbohydrates and 

 fats, occurs in modified amoebocytes. Excretory processes occur at the cellular 

 level, and soluble excreta and carbon dioxide are readily removed by the cur- 

 rents of water passing through the canal system. An abundant supply of 

 oxygen for respiration is provided by the same water currents. 



Responsiveness. The flow of water into the minute openings upon the sur- 

 face, through the canal systems and flagellated chambers to the spongocoel, 

 and out the osculum is the factor that conditions all other activities of the 

 sponge. This flow is maintained by the flagella of the choanocytes. The 

 steady and strong currents so produced, and the directed nature of the cur- 

 rents, have been found to depend on rather precise adjustments of the rela- 

 tive diameters of incurrent and excurrent pores and canals. In sponges, the 

 only easily demonstrable reactions to stimuli are the closing of pores and 

 oscula and contractions of entire masses of cells, which may temporarily 

 obliterate the canal systems. It is to be supposed that less violent reactions 

 to various stimuli involve slight constriction or expansion of pores and canals, 

 to bring about physiological adjustments of the volume and velocity of water 

 flow. The presence of special contractile cells surrounding water passages 

 has long been recognized (Fig. 9.12), but these were thought to operate as 

 independent effectors, responding directly to stimulation without the inter- 



Fig. 9.11. Transfer of ingested particles 

 from choanocytes to an amoebocyte. (Re- 

 drawn, after N. Pourbaix, from L. H. Hyman, 

 The Invertebrates: Protozoa through Ctenophora, 

 copyrigtit 1940 by McGraw-Hill Book Co., 

 Inc., printed by permission.) 



279 



