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THE AMEBIC AX NATURALIST [Vol. XLIII 



to the surface of turtles and fresh-water crustaceans and 

 are not true parasites, inasmuch as they feed upon small 

 animals in the water and not upon the vital juices of the 

 host. They probably form a connecting link between 

 turbellarians and trematodes, representing the first step 

 of the ancestors of the latter towards the acquisition of 

 parasitic habits. The next step was taken as the result 

 of the migration of the worms from the surface into the 

 mouth and cloaca and on to the gills, and then into the 

 internal organs, of the aquatic hosts. The worms thus 

 became true parasites. They learned to feed upon the 

 blood or the other juices of the host and were habitually 

 enclosed or immersed in its tissues and exposed to the 

 disintegrating action of its fluids. It is probable, as a re- 

 sult of these things, that the changes occurred which char- 

 acterize the body-covering of these worms. The integu- 

 ment which is common to most worms apparently would 

 not furnish a sufficient protection to animals thus situ- 

 ated, and it consequently came about in the course of 

 their evolution that the outer epithelium with its cuticula 

 was moulted or at least disappeared and the parenchyma 

 acquired the property of forming a thick cuticula-like 

 membrane on its outer surface to protect the animals 

 from the peculiar dangers of their environment. A pro- 

 tective function similar to this is, as Leuckart points out, 

 very frequently exercised by cuticula-like connective 

 tissue structures of various kinds throughout the animal 

 kingdom. 



Von Graff (1903) has made the observation, it is 

 interesting to note, that in certain of the parasitic turbel- 

 larians (Synccplidium) a process similar to this has evi- 

 dently gone on, for the animals have lost their integu- 

 mental epithelium together with its cilia and are covered 

 witli a cuticula similar to that of trematodes. 



The first steps in the formation of the cuticula have 

 been minutely observed, as already stated, in the trema- 

 todes by Looss (1892, 1903) and Roewer (1906) and in 

 cestodes by Young (1908). According to Looss, it first 



