THE TURBELLARIA 



All existing Platyhelmia, with a very few exceptions, are 

 hermaphrodite, and we are justified in attributing to the ancestor 

 a similar character. The male organs consisted of a pair of 

 tubular testes and of ducts, which unite posteriorly to form a 

 muscular copulatory organ capable of being protruded from a 

 median pore behind the mouth. Probably this penis was used 

 for perforating the soft body of another animal at any point, so 

 that special female receptive ducts were not at first necessary. 1 

 The female system consisted of masses of germ cells, derived from 

 the wall of the archenteron ; but very early in the history of 

 this group definite tubular ovaries, the walls of which are pro- 

 duced into special oviducts, were developed, for the liberation 

 of the fertilised ova; these ducts then united to form a special 

 sac for the reception of the penis (bursa copulatrix), while other 

 parts of the duct became dilated to form a spermatheca (Fig. II. 3). 



Of the descendants of this ideal ancestral Platyhelminth, some 

 have retained the ciliated epidermis and the free mode of life ; 

 while others have taken to a parasitic habit, and lose this ciliated 

 covering in the course of their developmental history, and are 

 covered by a " cuticle " in the adult stage ; in connection with their 

 parasitism special organs of fixation, suckers and booklets, are 

 developed in various parts of the body. The line of descent of the 

 free-living forms includes the Turbellaria and the Temnocepha- 

 loidea. The parasitic forms embrace two classes the Trematoda 

 and Cestoidea ; in the former the alimentary tract of the ancestor 

 is retained, while in the latter it has been entirely lost, and no 

 trace of it is presented at any period of the developmental history. 



CLASS I. TURBELLARIA (EHRB.). 



Order 1. Rhabdocoelida. 



Sub-Order 1. Rhabdocoela. 



Fam. 1. Macrostomidae. 



,, 2. Microstomidae. 



., 3. Prorhynchidae. 



4. Mesostomidae. 



5. Proboscidae. 



6. Vorticidae. 



7. Solenopharyngidae. 



Sub-Order 2. Alloiocoela. 



Fam. 1. Plagiostomidae. 



2. Monotidae. 



,, 3. Bothrioplanidae. 



1 Cf. Whitman, Journ. Aforph. iv. 1891, p. 386. 



