558 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Echinorhynchus, becoming closely fitted to its surface, and 

 apparently persisting throughout its entire life. The devel- 

 opment of the Echinorhynchus now approaches completion. 

 The lemnisci appear. Hooks arise on the surface of the pro- 

 boscis, not, as might be supposed, from its outer cuticle, but 

 from specially modified cells of an inner membrane. The in- 

 ternal organs begin to assume their final aspect. The ex- 

 ternal form of the adult organism is rather slowly reached,, 

 and a few changes which take place after transference of the 

 Echinorhynchus to its final host have yet to be observed. 



The Acanthocephala undoubtedly present certain resem- 

 blances to the Nema&oidea, and more particularly to the Gor- 

 diacea, but the fundamental differences in the structure of 

 the muscular and nervous system, and in that of the repro- 

 ductive organs, are so great, that it is impossible to regard 

 them as Nematoids which have undergone a retrogressive 

 metamorphosis. In their case, as in that of the Cestoidea 

 and that of the Dicyemida, it is, I think, desirable to keep 

 one's mind open to the possibility that anenterous parasites 

 are not necessarily modifications of free, enterate ancestors. 



THE DICYEMIDA. In 1830, Krohn discovered certain cili- 

 ated filiform parasites in the renal organs of Cephalopods, 

 to which Kb* Hiker subsequently gave the name of Dicyema. 

 Recently, these strange organisms have been made the subject 

 of renewed investigation by E. van Beneden, from whose 

 elaborate memoir 1 I take the following account of their 

 structure : 



The body of a Dicyema (Fig. 158, 1.) consists of one large, 

 cylindrical, or more or less fusiform, axial cell, which extends 

 from the slightly-enlarged head-end, by which the animal is 

 attached, to its posterior extremity, and is invested by a 

 single layer of relatively small flattened cortical cells. These 

 are arranged, like a pavement epithelium, around the axial 

 cell, their edges being juxtaposed ; they are nucleated, and 

 their free surfaces are ciliated. There is no interspace be- 

 tween the cortical cells and the axial cell, and the organism 

 is a simple cell-aggregate, devoid of connective, muscular, or 

 nervous tissues. 



The cortical cells which invest the anterior or head-end 

 of the Dicyema have peculiar characters, and are distin- 

 guished as the polar cells. They are arranged in such a 



1 " Recherches sur les Dicyemides." ("Bulletin de 1'Acad. Royale de Bel- 

 gique," 1876.) 



