THE ANENTEROUS INVERTEBRATA. 577 



The Tunicate Pharyngopneusta, with their caudate larvse, 

 may be supposed to stand in the same relation to the Turbel- 

 lariform Pharyngopneusta, as the Trematoda, with their cer- 

 cariform larvae, to the Turbellaria. 



Another very well marked division is that of the Echino- 

 dermata, the characteristics and relations of which have 

 been fully discussed in Chapter IX. 



Although the structure and development of Sagitta have 

 now been as thoroughly elucidated as those of any animal, 

 the proper Taxonomic place of the Choetognatha is still an 

 unsolved problem. The issues, however, appear to be nar- 

 rowed to these : either they belong to the Annelida, or to the 

 Nematoscolices, or to the Trichoscolices ; or the Chaito- 

 gnatha are to be regarded as an independent division, allied 

 to all these, and perhaps to the lower Arthropoda. I am dis- 

 posed to adopt the last view, chiefly on the ground of the 

 mode of development of Sagitta, which is unlike anything at 

 present known to occur in Annelida, Trichoscolices, JVema- 

 toscolices, or Arthropoda. 



The Acanthocephala are hardly less anomalous than the 

 Choztognatha. Taking into account the Gordiacea and the 

 characters of the proboscis in the Nematorhyncha, there is 

 undoubtedly room for the suggestion that they are specially- 

 modified anenterous JVematoscolices, and should be classed 

 among the latter. But here, as in the case of the Cestoidea, 

 there are many difficulties in the way of accounting for these 

 anenterous forms by the supposition that they are the results 

 of a retrogressive metamorphosis of enterate animals. 



This question of the true relations of the anenterous in- 

 vertebrates — by which I mean not only those which, like the 

 male Rotifers, have no functional alimentary canal in the 

 adult condition; but those which, like the Cestoidea and the 

 Acanthocephala, never exhibit a trace of an alimentary canal, 

 even in the embryo ; which is usually dealt with so summarily 

 by the assumption of retrogressive metamorphosis — acquires 

 still more importance, when we attempt to determine the 

 Taxonomic place of the Dicyemida. 



Prof. E. van Beneden has proved that these parasites can- 

 not be dismissed, sa?is fagon, as retrogressively metamor- 

 phosed " worms ; " and though I am not disposed to attach 

 much weight to the absence of a mesoderm, on which Van 

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