224 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



to the membranous piece which connects the basal 

 joints of the appendages with the walls of the thorax; 

 the third and uppermost set consists of pleuro- 

 foranclis, so-called from their attachment to the sides 

 of the thorax. Among different members of the 

 group we find a difference in the number of these 

 gills, and here, as elsewhere, in scientific investiga- 

 tions, much time is saved, and intellectual operations 

 considerably aided, by the use of formulae. The fol- 

 lowing table, taken from Huxley, may be regarded as 

 a type which is to be followed out when making the 

 dissection of any one of the higher Crustacea. 



HYPOTHETICALLY COMPLETE BRANCHIAL FORMULA. 



Pleurobranchs. 



1 4 



1 4 



1 4 



1 4 



1 4 



1 = 4 



1 4 



8 + 8 + 8 8 32 



All these gills have essentially the same structure ; 

 they consist of an elongated stem, within which 

 run two distinct canals, into one of which the blood 

 passes from the body, and by the other of which it 

 returns on its way to the heart ; connected with these 

 canals are a number of comparatively short hollow fila- 

 ments with thin walls ; the blood, therefore, on pass- 

 ing into them, is separated only by a thin membrane 

 from the oxygenated water that is passing through the 

 gill chamber, and an exchange of carbonic acid and 



* There are many reasons for beginning to count the segments 

 one farther back, and to call vi. what Prof. Huxley calls vii. 



