CHAPTER VI 



TIIK LOWER SILURIAN < >STRA< < >l>.\ 



MI\M->< >T.\. 



M i. 11.1:1 H 



Thi- order <>f Crustacea compii-e- -mill, generally minute, iniini.il- ha\ing the 

 entire l.o.|y em-lo-ed in ;| -hell or curapice con-i-ting of two more or le>- nearly 

 e.mal calcareou- or corneou- \uhe-. united along the Lack liy a ni.'iiilirane, and 

 capable of being opened at their ventral margin-. Tlie valve.- are clo-ed hy a Mih- 

 central adductor mu-c|e. the attachment of which i- marked on their inner -id. 

 a tn [ pit or a numKer of -mall -pot-. '1'he I tody i- not segmented Imt ha- 



seven pair- of appendages, of which ' two are antenna-, while they, like the 



other-, are al-o adapted for creeping ami -wimmiritf. These appendages, together 

 with the caudal extremity of the short abdomen, are protruded along the ventr.il 

 margin of the carapace when the valves are opened. 



lielmid the tir-t two pair- of appendage- (antennnle- and antenna-), i- a pair of 

 mandilile-, followed hy ;i pair of maxilhe, while the third and fourth pair- may l>e 

 either legs or j i\\-. I inally the last two pairs are leg-like and generally stronger 

 than the preceding pair-. The extremity of the alidomen may lie bifurcated or con- 

 -i-t of a -in^'le -pinous plate. 



As a rule the eyes are uell developed, with commonly a -mail median and two 



I'he position of the latter is often indicated on the 



exterior of the valve> Ky a -mall "eye tulienv ii-tinct heart may l>e present 



or ali-ent. Imt the alimentary and generative or_'.ni- are well developed. 



The n,tracoda. or " wa' t hey an- often called, arc repre.-ented l>\ 



nnmeron- form- hoth in fre^h water and in the sea. Of the families only the t'ljf-i 

 are chiefly fresh-water form-, while mo-t of tlie other families ai I to 



marine or hracki-h Taken as a whole they are to be considered a- -hallow 



water inhaliitants. and of -ocial halnt-. I.eing found in great numbers -wimmi 

 the -nrface of tlie water or creeping over the l>ottom. Kemain- of ()- abound 



