188 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Trematode type, in which the endoderm has got no further 

 than the spongy condition which it exhibits in Convoluta 

 among the Turbellaria, and in which no oral aperture has 

 been formed ; or, lastly, it is possible that the central cavity 

 of the body of the embryo Tmnia simply represents a bias- 

 tocoele. 



If the Cestoidea are essentially Trematodes, modified by 

 the loss of their digestive organs, some trace of the digestive 

 apparatus ought to be discoverable in the embryo tape-worm. 

 Nevertheless, nothing of the kind is discernible, unless the 

 cavity of the saccular embryo is an enterocoele. And if this 

 cavit}^ is a blastocrele, and not an enterocoele, it may become 

 a question whether the tape-worms are anything but gigantic 

 moruloe, so to speak, which have never passed through the 

 gastrula stage. 



