On the supposed Vertebration of the Tail in Appendicularia. 295 



end of the chord. This is what we sliould expect as from the po- 

 sition of that part of the muscle layer relatively to the chord it 

 is not subjected to the same strain during fixation as the remain- 

 ing i)art. Neither can that part, from its position at the tip of the 

 tail, be subjected to the same tear and wear during the life of the 

 animal. A break, in the musculature of this species, lying beyond 

 the posterior end of the chord I have only exceptionally found. 



When a very young 0. dioica is examined it will be found that 

 the tail presents on each side a linear series of cells, the mother cells 

 of the musculature. These so far as I have been able to observe do 

 not correspond in number to the muscle segments. I have not yet been 

 successful in procuring one of these young specimens from which 

 longitudinal sections of the tail could be made, but when examined 

 entire there appear to be about 12 cells in the linear series. In the 

 anterior two thirds of the tail the cells are large and regular, but 

 they are smaller and irregular in the posterior third. There is in 

 addition a single terminal cell which is capped by a cell of the epi- 

 thelium. There seem to be two of the large regularly placed cells in 

 the width of the muscle band. These evidently correspond to the 

 "muscle-cells" which Kowalewsky ') figures in his drawing of the 

 transverse section of the tail of the Ascidian larva. The arrangement 

 of the nuclear reticulations in the adult is evidently in agreement 

 with the arrangement of the cells in the young form, but so far as 

 my observations go the cells of the latter are not identical with the 

 muscle segments of the former. Hence as far as 0. dioica is con- 

 cerned, I do not see my way to accept Seeliger's view already 

 quoted, nor to agree with him in regarding each muscle segment as 

 a huge single muscle cell. The breaks in the muscle layers seem to 

 be simply fractures along transverse lines of weakness; the primitive 

 structure of the tail apparently having nothing to do with the location 

 of these lines. In all probability this applies also to F. furcata. 



The appearance of the anterior and central regions of the tail in 

 a very young O. dioica is shown in Fig. 10, PL 18. In a slightly older 

 specimen the longitudinal striation due to the formation of the muscle 

 fibres becomes visible, and it is to be observed that the fibres in one cell 

 correspond in position with those of the cells next in the series, even while 

 the boundaries of the cells remain distinct. The musculature is totallv 



1) A. KowALEwsKY, Weitere Studien über die Entwicklung der 

 einfachen Ascidien, in: Archiv Mikrosk. Anat., 1871. 



