i6 



HISTOLOGY 



Each animal lives at the end of its stalk independently of the others 



of the body. It secures 

 its food by means of its 

 own ciliary movements 

 and carries on for itself 

 all metabolic and repro- 

 ductive functions inde- 

 pendently of the colony 

 as a whole. 



An Example of a Col- 

 ony of the Second Order : 

 Gonium pec tor ale. A 

 specimen of Gonium 

 pectorale is always com- 

 posed of sixteen oval 

 cells attached laterally 

 to one another in such 

 a manner as to form a 



square colony with 



FIG. 14. Gonium pectorale, showing the individuals. (From 

 CALKINS after STEIN.) 



twelve cells on the margin of the square and four cells inclosed by the 

 lateral ones. The 

 entire colony is 

 surrounded by a 

 gelatinous sheath. 

 Each cell is oval 

 and inclosed within 

 a cellulose wall. 

 The cytoplasm con- 

 tains a cup-shaped 

 green chloroplast 

 and a centrally 

 placed nucleus. 

 Near the open mar- 

 gin of the chloro- 

 plast there is a 

 bright red stigma. 

 In the cytoplasm 

 opposite the open- 

 ing of the chloro- 

 plast are two 

 contractile vacu- 

 oles. From this 

 same region of the cell two slender flagella are given off (Fig. 14). 



FIG. 15. Volvox globator, a colonial organism of the third order. 

 (From WILSON, after J. H. EMERTON.) 



