360 ZOOLOGY FOR MEDICAL STUDENTS CHAP. 



so formed becomes continuous with the wall of the oviduct so that in 

 the teleost the eggs never pass into the peritoneal cavity at all. 



As regards the evolutionary origin of the male duct we obtain, as 

 will be seen later (p. 378), interesting hints from some of the more 

 ancient types of fish. 



The blood-system of the teleost is arranged on the same general plan 

 as that of the elasmobranch, the only difference that need be mentioned 

 being that the two posterior cardinal veins tend to become approximated 

 ther and to undergo fusion to form a single interrenal vein which 

 passes forwards between the two kidneys. An important and striking 

 difference in detail is seen in the structure of the heart. In the elasmo- 

 branch the portion of the primitive heart or cardiac tube lying between 

 the ventricle and the anterior boundary of the pericardiac cavity 

 constitutes the rhythmically contractile conus arteriosus. In a few 



aberrant teleosts such as the 

 Tarpon (Megalops) the mus- 

 cular, rhythmically con- 

 tractile, character has become 

 restricted to a comparatively 

 short portion next the ven- 

 FlG I50 tricle which, however, still 



Diagrammatic transverse sections illustrating the Contains tWO valvCS to 



B ar ca f Teleostean fishes> A> Perch> sent each of the original three 



longitudinal rows of pocket- 



valves. In the typical teleost the rhythmically contractile part of the 

 conus has disappeared entirely and the original three longitudinal rows 

 of pocket-valves are each reduced to a single valve, the three forming 

 a circle round the ventricular exit. The whole of the cardiac tube lying 

 between this and the anterior limit of the pericardiac cavity has lost 

 those features which entitled it to be regarded as physiologically a 

 chamber of the heart the presence of a thick layer of striped muscle 

 fibres in its wall, and the fact that it is rhythmically contractile. It 

 has on the other hand approximated in character to the ventral aorta, 

 its wall being composed for the most part of tough connective tissue 

 containing numerous elastic fibres. In the majority of teleosts this 

 anterior non-contractile portion of the cardiac tube is considerably 

 thi.kened forming what is called the aortic bulb 'differing from the 

 il muscular conus in its whitish colour. 



Students of human anatomy should take care not to be confused by the 

 MM bulhraa cordis which is applied in human anatomy to what comparative 

 iius arteriosus. 



