4 SCIENCE PROGRESS 



material, or contain digestive ferments. The ctenidia of a mollusc, 

 the adaptive gills of a limpet, the branchial leaflets, respiratory 

 plumes, cerata, etc., of other mollusca, the tentacles of a tube- 

 forming worm, the branchiae of an ascidian, and other structures, 

 are all supposed to be organs in which oxygen is absorbed from 

 solution in sea-water, and from which carbonic acid and other 

 waste substances are given off to the surrounding medium. When 

 special apparatus for the respiratory function does not exist, it is 

 usually supposed that this function is carried out by the entire 

 surface of the skin. Sometimes, as in the case of the lower 

 Crustacea, it has been stated that the pumping movements of the 

 rectum which may often be observed in these organisms are 

 concerned with the respiratory process, water containing oxygen 

 being taken into the alimentary canal, and absorbed by the 

 internal surface of the latter. " Eyes " are always visual organs. 

 So also with many investigations into the comparative physiology 

 of the invertebrata. Proteolytic, amylolytic, and lipolytic fer- 

 ments have been isolated from the alimentary canal and its 

 glands ; and since the reactions of these substances in vitro are 

 similar to those of the digestive ferments of the mammal, it is 

 assumed that the metabolic processes are also similar. 



Immediately we assume that the marine invertebrata possess 

 organs which carry out functions essentially similar to those of 

 the warm-blooded animal we encounter difficulties with regard 

 to their alimentation. It is true that many carnivorous species 

 which live in the sea may be very similar to the mammal with 

 respect to their general metabolism, but we cannot suppose 

 that this is universally the case with the free-living animals. 

 We find it usually stated, for instance, that the gills of a lamelli- 

 branch mollusc such as the mussel are organs which serve for 

 respiration, and also for the establishment of a current of water 

 into the mantle cavity, which current carries with it the food 

 organisms. These food organisms are strained out by the 

 meshwork of the gill filaments, are caught up by the labial 

 palps, and are taken into the mouth. Then digestion and 

 assimilation proceed presumably in much the same manner as 

 in the higher animals. I do not know of any estimations of the 

 respiratory surface of the gills of such a mollusc, but one is 

 impressed with its magnitude in comparison with the mass ot 

 the body of the animal. So also with an ascidian, where the 

 surface of the branchiae is a relatively large one. Yet these 



