148 AN INTRODUCTION TO ZOOLOGY 



carbonate of lime. Four varieties of spicules are always present, 

 (i) long straight monaxon rods guarding the osculum, (2) short 

 straight monaxon rods surrounding the incurrent pores, (3) tri- 

 radiate spicules always found embedded in the body wall, and 

 (4) T-shaped spicules lining the cloacal cavity; four- and five- 

 rayed spicules may also be present. Spicules are built up within 

 cells called scleroblasts (Fig. 73), which form part of the inner 

 stratum of the dermal layer. A slender organic axial thread is 

 first built up within the cell; around this is deposited the calcare- 

 ous matter; the whole spicule is then insheathed by an envelope 

 of organic matter like that composing the axial thread. 



Nutrition. Grantia lives upon the minute organisms and 

 small particles of organic matter that are drawn into the incur- 

 rent canals by the current of water produced by the beating of 

 the collar-cell cilia. Some of the food particles are probably 

 ingested by the porocytes; but the majority of them are engulfed 

 by the collar cells. Digestion, as in the Protozoa, is intracellular, 

 food vacuoles being formed. The distribution of the nutriment 

 is accomplished by the passage of digested food from cell to cell, 

 aided by the ameboid wandering cells of the middle layer. 



Excretory matter is discharged through the general body surface, 

 assisted probably by the ameboid wandering cells, and possibly 

 by the collar cells also. Respiration likewise takes place, in the 

 absence of special organs, through the cells of the body wall. 



Sponges do not possess differentiated nervous organs, but are 

 able to respond to certain stimuli. The pores and oscula are 

 surrounded by contractile cells, called myocytes, which are able 

 to close these openings. Apparently these cells respond to direct 

 stimulation, since no nervous tissue is present. They, therefore, 

 represent what may be considered the very beginning of a neuro- 

 muscular mechanism (130, pp. 59-60). 



Reproduction. Reproduction in Grantia takes place by both 

 sexual and asexual methods. In the latter case, a bud arises 

 near the point of attachment, finally becomes free, and takes up a 

 separate existence. 



