BIOCHEMICAL EVOLUTION 341 



digestion based on the secretion of enzymes into the lumen of the digestive 

 tube. In general, Lamellibranchs feed by a ciliary mechanism which is 

 responsible for the collection of fine particles, mainly of phytoplankton. 

 The only extracellular phase of digestion they have is in the action of an 

 amylase, all the other enzymes acting intracellularly. Among the herbi- 

 vorous Gastropods (the Pulmonata excepted), like Yonge we can distin- 

 guish two groups : those possessing a crystalline style and those which do 

 not. In the former, as for example the Streptoneura, conditions are very 

 similar to those found in the Lamellibranchs, amylase being the only 

 extracellular enzyme, the digestive diverticulae acting as organs of absorp- 

 tion and intracellular digestion, but never of secretion. The second group 

 of herbivorous Gastropods, those not possessing a crj^stalline style, as is 

 the case for the Tectibranchs and the Nudibranchs, show considerable 

 diversity and in certain cases there is a proteinase in the juice in the 

 digestive tube. When we come to the carnivorous Gastropods such as 

 Murex, a proteinase actively secreted by the digestive diverticulae is 

 always found in the digestive tube. Among them, too, there is also 

 intracellular digestion. In addition, the salivary glands secrete amylase. 

 Among the Pulmonata, such as the snail, the hydrolytic processes are 

 almost completely extracellular. Only protein hydrolysis is intracellular. 

 In the Cephalopods, digestion is exclusively extracellular and intra- 

 cellular digestion has disappeared. 



(b) Specialisation by Acquisition of a New Constituent as a 

 Result of Molecular Evolution 



During the course of the evolution of the cells that contain it, an enzyme 

 system may become the object of a specialization of a new type. Snakes, 

 for example, do not mix digestive secretions with their prey by a process 

 of mastication. They swallow their prey after having injected it with a 

 secretion which initiates hydrolysis. In the least specialized form, for 

 example in Colubridae opisthoglyphae, a simple secretory tooth appears 

 at the rear of the upper jaw and serves for the injection of a secretion whose 

 function is purely digestive. In more specialized forms, this organ, 

 following a decrease in length of the maxilla, approaches the anterior part 

 of the buccal cavity and becomes an aggressive and defensive organ, as is 

 the case in Colubridae proteroglyphae and even more so in the Viperidae. 



The digestive origin of the secretion is further borne out by the presence 

 in snake venoms of such hydrolases as proteases, peptidases, phosphatases, 

 sterases, and lecithinases. The new specialization expresses itself by the 

 presence of hyaluronidase, assuring the diffusion of the venom, and by the 

 presence of substances of high toxicity (see Zeller, 1948). 



Another example of modification of an old system by the addition of a 

 new component is the urea-synthesizing system in the hepatic parenchyma 



