296 JAMES RANKIN, 



different from that of Aniphioxiis where each myotome has its own set of 

 independent muscle fibres, permanently separated from those of the 

 adjoininjj; myotomes by the connective tissue substance. In specimens 

 older still the continuity of the fil)res becomes established and the 

 cell boundaries disappear. In an adult 0. dioica there may be as 

 many as 160 fibres in the width of the broadest part of the muscle 

 layer. 



There are other breaks in the musculature besides those already 

 mentioned. Of most common occurrence are little "islands" of muscular 

 tissue which are frequently seen in tails of 0. dioica and 0. copho- 

 cerca. I have not observed them in F. furcata. They are shown in 

 Fig.sö, PI. 18. Sometimes breaks similar to those which form the 

 "islands" are seen on the margin of the muscle layer cutting off a 

 more or less crescent shaped mass. Probably these phenomena are 

 due to pathological changes in the muscles as granular material is 

 often seen at the ends of the fibres. This is shown in the middle 

 "island" of the figure. I have not observed any evidence of such 

 changes at the margins of the transverse ruptures. It may be re- 

 called that in certain pathological conditions ruptures occur of the 

 fibres of some human muscles, as, e. g., the rectus abdominis, adductors 

 of the arms. etc. 



When the animal is dying the tail is usually the first part to 

 give way. I have observed specimens where the body was quite active, 

 as shown by the movements of the cilia, while the tail was reduced 

 to a shapeless mass. Possibly the early decay of the tail may have 

 some bearing on the greater prominence of the breaks in the older 

 tails, and on the appearance of the "islands". 



Lankestek states that "the metamerism of the tail of FritiJ- 

 laria, though it may be viewed as an incipient formation of vertebral 

 segments, appears to be most satisfactorily explained as a remnant 

 of a more fully expressed 'vertebration' which was once possessed by 

 a larger and more elaborate ancestor of the Appendiculanœ, of which 

 existing forms are the reduced and degenerate de.scendants". In the 

 structure ot Appendiculnria, and in its dcveloj)ment so far as it is 

 known, there is little or nothing to support the theory of degeneracy ; 

 there is in its life history no evidence of descent from a more highly 

 develoi)ed ancestor. The invertebrate features shown by the animal 

 rather point to an invertebrate ancestry ; lead rather to the conclusion 

 that these organisms are in process of being conformed to vertebrate 

 Structure, and only show a remarkable middle stage between the two 



