746 COMPARATIVE PHYSIOLOGY. 



of the body, and that it is not limited to the alimentary 

 tract. Diastatic ferments are very frequently present. 



In Arthropods, peptic, tryptic, and diastatic ferments are 

 common. The peptic ferment is uniform throughout the 

 group, and has been termed " homaropepsin," to indicate 

 that it differs considerably from the pepsin of Vertebrates. 

 On the other hand, the tryptic ferment is not distinguishable 

 from that of Vertebrates. Both peptic and tryptic ferments 

 are often secreted by the same gland. The reason for this 

 and its physiological consequences are unknown. In the 

 Crustacea the reduction of the digestive mid-gut is com- 

 pensated for by the increasing development of the digestive 

 gland, which is in reality a compact mass of caeca. In the 

 caeca digestion and absorption go on, fats only being directly 

 absorbed by the short mid-gut. 



In the Mollusca, oesophageal glands, usually called 

 "salivary," are very common, and often large. In some 

 cases, as in Dolium and others, these glands secrete only 

 mineral acids (sulphuric in Dolium). According to Bunge, 

 these acids, like the hydrochloric of the Vertebrate stomach, 

 have chiefly an antiseptic action, destroying Bacteria intro- 

 duced with the food. If this be correct, the advantage of 

 the oesophageal position is very obvious. The true digestive 

 gland of Molluscs is the " liver," which is usually very large, 

 and often secretes diastatic, peptic, and tryptic ferments. 

 Its secretion, like the perivisceral fluid, is always neutral or 

 slightly alkaline. Peptic digestion may be rendered possible 

 (i) by the presence of acid derived from the oesophageal 

 glands, or (2) by the acid nature of the food; but nothing 

 is known with certainty. 



The nutrition of the tissues. — After the complex food 

 substances have been broken down into simpler ones, they 

 must be carried to the tissues, there to be employed in 

 repairing waste, or in growth. In a simple Protozoon there 

 is no difficulty ; like a primitive community, the single cell 

 supplies its own wants, and the question of transport is 

 never raised. In a Metazoon, on the other hand, as in a 

 civilised state, there is much division of labour, and the 

 question of the transport of manufactured material becomes 

 very important. 



In a Vertebrate the blood is the great transporting agent ; 



