300 
RESPIRATION. 
to cooler (Quarters aiuoiig damp moss or herbage, etc., where, owing 
to evaporation, the temperature may he as mucli as 20° below that 
indicated l>y a neighbouring thermometer. 
Every species ami every individual, however, naturally seeks those 
conditions most agreeable to its own welfare, and it is probable that 
in correlation with this habit there is for every species a special 
o[)timiim external temj)eraturc and also an oj)timmn pulse-rate, 
especially suited to the action of its organs and to the re(piiremeiits 
of its economy. Some species, like Hcliv pisuna and Helix eartmlana, 
habitually exi)o.se themselves to the full glare of a burning sun and 
have prohalily a high optimum pulse-rate; others, which secrete them- 
selves during the heat of the day in moderately dry and sheltered 
spots, attacheil to stems or undersides of leaves in hedgerows, have a 
l)ulse-rate varying between 3') and GO; while species which habitually 
hide themselves in wet moss or at the roots of damp herbage range 
between 2') and 43 per minute. 
The .\(^uiEERors or water-vascular system in the mollusca is most 
develojted in the marine Pelecypods, some species possessing a coni- 
plete network of ramifying canals within the foot, which, by the 
ince])tion of water from ivithout, render the foot turgid and linn, 
supplementary to the tiirgescence due to the retention of the blood 
therein. 
Absorbent or minute i)ore canals opening to the e.xterior have also 
been allirmed to exist in our Eritish Unionidmand S]i)luerii(he and are 
stateil to have been observed even in Helix jKiniatid and other 
terrestrial sj)ecies, the Limaces being specially noticeable for the rapid 
absorption of moisture by their integument. 
Respiration is a jimcess vital to aiiinial existence, and in the lower 
forms of life may he accomplished simply by an interchange between 
the surface of the body and the surrounding medium, hut in the more 
highly organized animals this interchange between the gases of the 
blood and tissues is chielly iierformed by specialized and suitably 
lilaced organs, whose res[)irations or breathings, like the pulsations of 
the heart, vary in nundier in different sjiecies and also differ accord! ng 
to temperature, and the age, amount of exertion, etc., of the 
individual, but are always much more slowly and irregularly per- 
formed, the energy and activity of the organism being in proportion 
to the amount of oxygen absorbed. 
