294 JAMES RANKIN, 



not enable me to distinguish such". 1 think it may be safely con- 

 cluded that no such separating membrane or septum exists. I have 

 examined many specimens of tails mounted Nvhole and of loii^ntudinal 

 and transverse sections, but have found no trace of an intervening 

 connective tissue substance. The transverse markings of the tail are 

 simply ruptures of the muscle fibres and nothing more. A closer exa- 

 mination of these ruptures will make their accidental character more 

 evident. Frequently I have found as in Fig. 4, PI. 18, that the break 

 extends only half way across the muscle band; sometimes, though 

 rarely, there may be a break on one side of the tail and no corre- 

 sponding one on the other ; in a few cases the break is clear and 

 decided, but often it is so faint that it is hardly visible at all and 

 only discoverable by a careful examination. In this last case only a 

 few fibres here and there may be ruptured, the remainder l)eing per- 

 fectly intact. In many cases, chiefiy in F. furcata, as shown in Fig. 1, 

 PI. IS, the continuity of the fibres is preserved from one end of the 

 muscle layer to the other. In most instances in 0. dioica where a 

 complete break occurs it is of the character shown in Fig. 4, PI. 18, 

 but very often the ends of the ruptured fibres interdigitate irregularly 

 for considerable distances. It is evident to me that the breaks in the 

 musculature are not simply demonstrated by the preparation but caused 

 by it. If tendinous intersections were present I think it is probable 

 that they would form the strongest and not the weakest part of the 

 musculature. During my stay at Millport 1 found that when living 

 specimens were examined under the microscope they remained alive 

 only for a few minutes. While they were in a dying condition the 

 behaviour of the tail appeared to throw some light on the origin of 

 the ruptures in question. The muscle fibres contracted but the elastic 

 axial chord did not yield as it was being pulled on both sides, the 

 result being that the muscle layers gave way in one or more places. 

 They did not simply break right across but gave way bit by bit, 

 as the strands of a hempen cord would do if it were slowly sub- 

 jected to a breaking strain. 



The number of the muscle segments resulting from the inter- 

 ruptions in the fibres appears to be remarkably constant. Lankksteu 

 states that there are 7 in F. furcata; Seetjoeu that there are 10. 

 I have not found a sp(!cimen which enabhîd me to determine the 

 nun]i)er independently In well marked tails of 0. dioica there are 

 usually y. Occasionally a ninth fracture (thus making 10 segments) 

 is found in that part of the musculature lying beyond the posterior 



