M ETAZOA VERMES. 317 



787). Borlasia unlike Meckelia tapers at both ends, 

 making it difficult at first sight to tell which is the ante- 

 rior region. These worms possess the same general 

 characters as Cerebratulus, though, as compared with 

 this giant, they are diminutive members of their class. 



TURBELLARIA. 



The Nemertea and Turbellaria have been considered 

 by many naturalists as the representatives of the ances- 

 tral forms of not only the Platyhelminthes or group of 

 Flat Worms, but of the whole subkingdom of Vermes. 

 The reasons why they are here regarded as derived rather 

 than primitive forms are the following: 



First : The trochophore stage of development is not 

 found in the Turbellaria nor in other Platyhelminthes. 

 Now, since this stage occurs in the early development of 

 many classes of animals and in the Annelida, as we have 

 already seen, we consider it of great phylogenetic impor- 

 tance. 



Secondly: The Turbellaria (excepting the Polyclads) 

 are either fresh-water or terrestrial animals, and, there- 

 fore, show in their structural features and mode of devel- 

 opment the specializations peculiar to animals with such 

 a habitat. -The rest of the class of Platyhelminthes are 

 extremely specialized by the suppression of parts. In 

 fact, this class offers some of the best illustrations of 

 specialization by reduction to be found in the whole ani- 

 mal kingdom. 



Thirdly : While it is true that most of the Turbellaria 

 are unsegmented in both the young and the adult condi- 

 tion, nevertheless there are members of the class which 

 are segmented when young and which lose this feature on 

 approaching maturity. This tends to prove not only that 

 the Turbellaria and Platyhelminthes belong to the great 

 group of animals with segmented bodies, but also that 



