TISSUE RESPIRATION. 
307 
the wliole respiratory function, as in tlie cold abyssal depths of the 
Lake of (leneva, where several minute species of Liinncm permanently 
dwell, their ^lulmonary sacs filled with water, and aipiatic respiration 
taking place by the lung cavity and by the integument, a prolonga- 
tion of the acpieous respiration of the embryo and newly-hatched 
young of the aij[uatic Pulmonates. 
During winter, when the pools are frozen and access to the air cut 
off, various species of Liminvtdw may often be observed beneath the ice 
crawling actively about and probably mainly respiring by their skin. 
Respiration by the skin is assisted by the presence therein of 
superficially placed coloring matters with an affinity for oxygen, as 
e.xem])lified in the orange-coloured foot of many Pelecypods, this 
colouring being usuallj' due to Tetronerythrin, a substance comliining 
readily with oxygen. The integument of Limax etc., has 
also yielded pigments which, in addition to other duties, are also 
functionally respiratory. 
Tissue Respiration is greatly developed in mollusca and is chiefl}" 
performed by means of the varied pigments so plentiful in the 
molluscan organism, and although their chief function is doubtless 
respiratory, as they combine with and retain oxygen within the 
system, yet they have also other functions, as in perfecting protective 
resemblances or acting as screens to underlying cells. 
These pigments, known as Enterochloro})hylls and Enterohaunatins, 
were first discovered by Dr. Sorby, and apparently contribute to replace 
hmmoglobiu in the Invertebrate system. 
The Enterociilorophylls (evrepov, gut ; green ; 
a leaf) found in the digestive gland are identical with the hepato- 
chroniates, and occur dissolved in oil globules and in the pi'otoplasm 
of the secreting cells, though also found in a granular form. The}’ 
are probably formed by the action of the digestive fluids upon the 
chlorophyll of the food, and eventually become lodged within the 
digestive gland and other organs. 
The Enteroh.ematins (Lwepor, gut ; atfia, blood) also occur chiefly 
in the digestive gland, though present in various other parts of the 
body. Myoh.ematin (pis, muscle ; afpu, Idood) is the true intrinsic 
coloring matter of invertebrate muscle, while IIistoirematin (urTos, 
tissue ; aipa, blood) is characteristic of organs and tissues, but both 
may be reinforced or replaced by hannoglobin where extra activity 
of internal respiration is reipiired. 
