63 



of to the perivisceral fluid, which also takes care of the distri- 

 bution of the food. 



Real resorption though in its kind primitive takes place here 

 in that way. ,,L'assorbimento e dunque necessariamente nelle 

 Oloturie un processo fisiologico." This is still more evident 

 from the following set of experiments. Enriques demon- 

 strated that sugar can be resorbed from solutions of equal 

 osmotic pressure or even lower. For this purpose he prepared 

 artificial salt-sugar solutions, the osmotic pressure of which he 

 controlled by freezing point determinations. The osmotic exchange 

 takes place chiefly by means of water passage, the chlorides do 

 not pass as quickly. In one of Enriques' experiments as much 

 as 2.3 c.c. of water had passed over to the outside and only 

 0.004 gr. of NaCl into the gut. As a rule such osmotic diffe- 

 rences as are used in the experiments will not occur in nature. 

 Thus we can conclude that the Holothurian intestine possesses 

 as Enriques calls it, an ..absolute semipermeability", and is not 

 capable of allowing either salts or food substances to pass 

 through by ,,diosmosis". To quote his own words: ,,La mem- 

 brana intestinale presenta dunque in modo palese due proprieta 

 apparentemente contradittorie : impermeabilita per diffusione, 

 permeabilita per assorbimento." A true ^physiological" resorp- 

 tion occurs the mechanism of which, as in Vertebrates, is 

 as yet completely unknown. 



This resorption question is one of the big problems of general 

 physiology and it is to be hoped that in the next decennia com- 

 parative physiologists will study the genesis of this remarkable 

 property of the living gut wall more in detail. It is in the various 

 groups of invertebrates that we must observe it at its origin. It is 

 by no means primitive : in the lower forms (Sponges, Coelen- 

 terates, Plathelminthes, etc.) the ,,assimilation" of the food occurs by 

 means for phagocytosis, later on we find special organs developped 

 for resorption (as the ,,Mitteldarmdruse" of Crustaceans and Mol- 

 luscs, the ,,liver" of the starfishes see the next chapter 

 the coeca of Aphrodite, etc. etc.) and only in the higher invertebra- 

 tes do the first traces of resorption over the entire gut wall occur. 

 In the lower forms the gut is just as long as is necessary for a safe 

 conduction of the food to the organ of enzyme secretion and resorp- 

 tion and for a rapid elimination of the waste. Frequently it is even 

 covered by completely impermeable substances, such as chitin. 



Only in animals in which the gut begins to be much 

 longer, may we expect to find the solution of this big problem. 

 And even in many of these three characteristics of this function 

 are absent: 1. the polarity of the intestinal membrane, the 

 resorption of fluid together with the food substances and . the 

 rapidity of the process, as Jordan states for the case of Helix. 

 The study of the gradual development of these three properties 

 in the different groups - in Helix e. g. sugar can be taken up 



