740 COMPARATIVE PHYSIOLOGY. 



voluminous caeca are not areas where digestion goes on, but, 

 as in Aphrodite , merely reservoirs for the secretion. In the 

 Holothurians no digestive glands have been, as yet, found in 

 connection with the gut, nor can any ferment be extracted 

 from its walls. The contents of the gut are, however, mixed 

 with a peptic ferment ; this can also be extracted from 

 extra-intestinal parts of the body, so that ferment secreting 

 glands must exist. A similar diffuseness in the occurrence 

 of ferments is very common among the Echinoderma. It is 

 therefore asserted that digestion must go on in various parts 

 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 Mollusca, cesophageal 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 Doliuni). 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 cesophageal 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 cesophageal 

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

 known with certainty. In the Eolidae, the gut gives off 

 prolongations which pass upwards into the dermal papillae. 

 Into these the contents of the alimentary canal pass, and 

 here both digestion and absorption take place. They thus 

 become filled with chyle, which is directly absorbed by the 

 tissues. 



In Ascidians, Krukenberg was in many cases quite unable 



