ELEMENTARY VITAL PHEXOMEN'A 169 



and stars in echinoderms and sponges, are formed withm the cell 

 itself, and only after thej' have reached a certain size are thej- 

 given off to the outside by the customary mode of excretion of 

 solid bodies (Fig. 61;. 



AravJja shows best the mode of excretion of substances that lie 

 in the interior of the cell as solid masses. It has been seen that 







'"-Ih 



Fig. -jO. — Cruss.section ot bone. The- Fig. fX>. — Hyaline cartilage. Between tlie indi- 



compact ground-Kiib^t-iiioe lit.-^ vidiial eells a -solid, hyaline ground-substance 



between the star-shijjed ?x;ne- ha.s been excreted. (After Hatschek.) 

 cells. In the midd]'.- of the sec- 

 tion i-s a cross— e'tioii of a bone- 

 canal. (Alter Ilat-thek.) 



in the ingestion of food by AnivXa the food-ball enclosed in a 

 food-vacuole lies finallj- within the protojilasm. In this vacuole, 

 which may be termed a digestive vacuole, all digestible substance 

 becomes dissolved, and passes into the protoplasm ; but the indigest- 

 ible residue, such as shells of algae and of diatoms and the chitinous 

 masses of rotifers, remain in the vacuole, and become excreted 

 in the following manner: By the creepjing of the Aniala the 

 digestive vacuole in the streaming protoplasm comes tri lie very 

 near the surface, so that its contents are separated from the 

 medium inw'iily by a thin delicate wall of pirotoplasm. In such a 

 case the wall break'^ '^'&iy I'asily by the protoplasm flowing in all 



^-■gj ':§/ -^. # 



Fig. 01. — Formation of a triradiate calearcou^ star in an echinodcrm cell. (After .Semon.) 



directions away from the thinnest pilace, and the contents of the 

 vacuole togx-ther with the solid mass are i.-mptied to the outside 

 (Fig. 62). This moile of n.'inoval of solid constituents from the 

 protoplasm i^ found exclusively in cells that do not possess a 

 membrane, and hence chiefly in amoeboid cells of all kinds. 



A transition between the method of output of liquids and that 

 of solids is repn.-seiited by the secretion of mucus. The mucous 



