10 



INTROnrcriON TO ZOOLOGY 



whilst iinother foini, with shorter pseudopodia and several 

 nuclei, is distinguished as Amnehii, quartti. 



Uiiicellnlar Fornix vhlch secrete a Sluil. 



Many forms closely allied to Amoelja have taken one step 

 upward' on the ladder of life, liy acquiring the hahit of 



^ 4 scci'etin 



round their 

 hodies a covering, which 

 is not temporary as in 

 the case of the Amoeba's 

 "cyst," but is permanent, 

 and is of such a form, that 

 Avdrilst allowing all vital 

 processes to continue un- 

 checked, it seiwes as some 

 slight pi'otection to the 

 soft protoplasmic Ijody. 

 These protective "shells" 

 arc usually of calcium 

 carl)Oiiate (chalk), or of 

 silica, and they are per- 

 forated by one or more 

 pores through which the 

 protoplasm can freely 

 project; incidentally they 

 exhibit a wonderful 

 variety and beauty of 

 foim. 



Formation ^ ^'^^'^ '-'^ these are shown in Fig. 2. The 

 of Chalk special forms illustrated are all taken from sea- 

 Rock, water forms, which, when alive, float freely in the 

 Avater, but at death sink slowly to the bottom of the sea, 

 where their chalky shells may accumula,te, year after year, for 

 many centuries, forming a thick layei', such as is now being 

 deposited on the bed of the Atlantic Ocean, where this is not 

 at a greater depth than 12,000 feet.' 



^ At a greater depth tliaii this, only sihceous shells are foinid, for tlie 

 delicate calcareous shells Ijecome dissolvcil hy the carhoiiic acid in the water 

 which has a greater solvent jjower at the increased jiressHre resiiltiirg fi'oni 

 the greater depth below the surface. 



Fifi 



-Shells from a Chalk Formation. 



