On the supposed Vertebration of the Tnil in Appendicularia. 293 



a bilateral structiiro in the anterior two thirds of the tail as if the 

 nuclei of two collateral cells had become reticular and had partly 

 fused. In O. dioira, Ki«,'. »5, PI. 18, a similar reticulation is seen, also 

 lyinj^' in the f^n-anular protoplasm , but here the structure is much 

 ojjcner and apparently continuous from end to end so that separate 

 nuclei cannot be distinguished It also presents anteriorly a bilateral 

 arranj^ement. There seems to he no reason to doubt that these reti- 

 culations are derivetl from the nuclei of the primitive mother cells of 

 the tail musculature. In transverse section, Fig. 9, PI. 18, the cut 

 ends of these reticulations are seen as minute specks which stain more 

 deeply than the rest of the proto])lasm. 



The deeper part of the musculature consists of fibres which are 

 rectangular in section ; the longer diameter of the rectangle being 

 placed perpendicularly to a plane passing longitudinally from the 

 primitive dorsal to the primitive ventral surface of the tail through 

 the axis of the chord. The fibres lie side by side Uke laths placed 

 with their broad surfaces towards each other, or like the "trackers" 

 of an organ. As I have previously pointed out, interruptions in the 

 continuity of the muscle fibres are rarely, if ever, seen in living 

 specimens: at least I cannot be certain that I have ever detected 

 such in any of the numerous individuals which I have examined. As 

 is well known , however , in specimens which have been prepared in 

 the usual way (fixed by picro-sulphuric acid, mercuric chloride etc. and 

 stained), many transverse breaks are frequently seen in the muscu- 

 lature. These ruptures are not readily found in very young examples, 

 at least of 0. dioica. In the tail of F. furcata from which Fig. 1, 

 PI. 18, was drawn none were found even by careful search under great 

 magnification. In other examples of the same species I have observed 

 them only irregularly, most frequently not at all. It would thus ap- 

 pear that these interruptions are certainly not of constant occurrence 

 even in specimens of the same genus, which have been prepared in 

 exactly the same manner. In 0. coj^hocerca, of which, however, I have 

 only seen a few examples, I have not found a regular series of such 

 breaks. The muscle segments resulting from these ruptures Lan- 

 KE.STEK regards as myotomes "identical with those of Amphioxus". 

 In his paper in the Quart. Journ. of Microsc. Science he states that 

 he *'could not trace a distinct fibrous septum separating the myomeres 

 from one another, but merely a break in the continuity of the mus- 

 cular fibres", and that "it is probable that a very delicate membrane 

 separates each myomere from its successor, iiut my specimens did 



