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 Turhellaria^ and in which no oral aperture has 

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

 of the body of the embryo Toenia simply represents a blas- 

 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 

 cavitj' is a blastocoele, and not an enterocoele, it may become 

 a question whether the tape-worms are anything but gigantic 

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

 gastrula stage. 



