Od the supposed Vertebration of the Tail in AppiiKÜcularia. 291 



The main features in the structure of the tail being well known 

 I shall not again describe them. If true segmentation were present 

 we might find corresjjonding evidence in the axial chord (though not 

 probably in such a simple form as this), but we should certainly 

 expect it in the nerve cord and in the musculature. 



The axial cliord , according to Fol, "n'est autre chose qu'un 

 tube fermé des deux bouts, et rempli d'une masse de con- 

 sistance cartilagineuse". — " La paroi du tube est une mem- 

 brane très mince et anhiste. A sa surface interne, l'on découvre 

 des cellules lenticulaires, rarement étoilées, aplaties, et qui ne 

 montrent i)lus trace de nucleus". In 0. dioica I bave not found the 

 '"cellules étoilées", but in my preparations there is a distinct nucleus 

 in the lenticular cells. This and the relative position of these cells 

 are shown in Fig. 6 and 7, PI. 18. I have not observed similar nucleated 

 cells in the axial chord of F. furcata , where api)arently only the 

 disc-like nuclei have persisted. In neither case do the cells of the 

 chord correspond in number or position to the primitive muscle cells. 

 Their relative distribution in a very young form of 0. dioica is shown 

 in Fig. 10, PI. 18, where the cells have not yet begun to secrete the 

 cartilaginous substance. Fol draws attention to the firmness of 

 the chord. He says "On peut couper la corde en travers, la serrer 

 dans un compresseur, sans que cette masse change de forme, ou sorte 

 du tube qui la renferme". This elasticity I believe to have an im- 

 portant bearing on the phenomena observed in the musculature. 



The nerve cord of 0. dioica is a thin flattened filament closely 

 applied to the side of the axial chord. At the base of the tail is the 

 large caudal ganglion and I have counted from 13 to 15 of the smaller 

 so-called ganglia in this species. It is difficult to determine their 

 exact number as those near the posterior extremity of the tail consist 

 simply of single nerve corpuscles which are not readily distinguished 

 in prepared specimens. They are easily seen in living animals, but 

 the movements of the tail render it difficult to observe the precise 

 number. Fol states that "un fin canal parcourt le nerf et les deux 

 ganghons dans toute leur longueur". The sections which 1 have pre- 

 pared of 0. dioica show that the two ganglia (i. e. the cerebral and 

 the caudal) as well as the nerve cord in the body and tail, are solid 

 structures. By suitable staining I have been able to demonstrate that 

 the nerve cells on the caudal filament are in structure similar to those 

 in the caudal ganglion, which have a granular i)rotoplasm and a large 

 vesicular nucleus with a nucleolus , although Fol says concerning 



