TYPE PLATTHELMINTHES. 143 



tera only through the Sterrula ancestor common to both, or more probably 

 through an ancestor in which the mouth had developed, as well as a slight 

 differentiation of muscle-flbres, but in which no hollowing out of an enteron 

 had yet occurred. This appearing in a primitive acoelan form gave rise to 

 the Alloioccela from which two divergent lines of descent arose, cue leading 

 to the Rhabdocoels and the other to the Triclads and Polyclads. 



If this be the true phylogeny of the class, some evidence of it ought to 

 be found in the embryological history of some of the higher members of 

 the group in accordance with what is termed the Biogenetic law, which is 

 to the effect that an individual in its development recapitulates more or 

 less accurately its phylogenetic development, or, to put it more briefly, the 

 ontogeny is a recapitulation of the phylogeny. Secondary modifications, 

 especially in the form of the abbreviation or omission of certain stages, 

 may intervene in the individual development, forming what are termed 

 cenogenetic modifications, but notwithstanding exceptions produced in this 

 way the law is of general application. 



In 8tylochus the young larva is a solid body without any enteron and 

 represents, therefore, an Acoelan stage of development ; later the central 

 mass becomes hollowed out to form an enteron whose walls are not at first 

 clearly marked off from the surrounding parenchyma, and a representa- 

 tion of the AUoiocoelan condition results, from which the Polyclad condi- 

 tion gradually develops. Consequently in Stylochus the ontogeny indi- 

 cates a primitive nature for the Acaela, and agrees with the phylogeny 

 which has been outlined above. It must be recognized, however, that all 

 reconstructions of the phylogeny of the Turbellaria and all views as to 

 their affinities to the Cnidaria must be accepted with much reservation, 

 until the much-needed facts as to the developmental history of the Acoda 

 and Alloioccela are available. 



II. Class Teematoda. 



The Trematodes or Fluke-worms are throughout parasitic 

 either upon the exterior of their hosts or in the cavities of 

 their body, and in correspondence with this mode of life 

 structures are developed by means of which they adhere to 

 their hosts. These structures are of two kinds ; in all suckers 

 are present consisting of cup- like depressions whose walls are 

 richly supplied with muscle-cells, by the contraction of which 

 a vacuum is formed, and in many forms, in addition to these, 

 chitinous hooks occur. The suckers vary in number from one 

 (Monostomum) or two {Distomum, Fig. 76,) to several {Poly- 

 stomum), and at the bottom of one situated at the anterior 

 extremity of the body is the mouth-opening. This leads 



