740 COMPARATIVE PHYSIOLOGY. 



I voluminous csca 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 

 I 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 

 I tract. Diastatic ferments are very frequently present. 



In Arthropods, peptic, tryptic, and diastatic ferments are 



icommon. 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 



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



and its physiological consequences are unknown. 



In the MoUusca, oesophageal glands, usually called 



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



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



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



I these acids, likejhe hydrochloric of the Vertebrajte stomach, 



! have chiefly_aii_jintisepUc ac^tion, destroyijig^Bacteria intro- 



\ duced with the food. If this be correct, the advlintage of 



/I the cesophageal position is very obvious. The true digestive 



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



I 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 Eolidee, the gut gives off 



prolongations which pass upwards into the dermarpapillre. 



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 



