372 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Lepidosteus only the hind end of this persists as a short vestige. In the 

 Sturgeons again the anterior portion of the cardiac tube retains the form 

 of a rhythmically contractile conus arteriosus containing longitudinal 

 rows of pocket-valveSj while in Amia the rhythmic contractility and the 

 presence of pocket-valves has become restricted to a comparatively short 

 Stretch at the ventricular end, thus leading up to the condition seen 

 in the modern teleost (p. 360). 



The tubular connexion between the pericardiac and peritoneal cavities 

 existing in the Elasmobranch is to be found also in the Sturgeon. 



Most species of Acipenser are anadromous, but the other ganoids 



are permanent residents in 

 fresh water. The larvae 

 possess cement-organs simi- 

 lar in their nature to those 

 of Polypterus. 



The three genera Lepi- 

 dosiren, Protopterus and 

 Ceratodus — grouped to- 

 gether under the name 

 Dipnoi — are the last sur- 

 vivors of an extremely 



j» 



B 



Fig. 159. 



Mitosis in Lepidosiren — A, and a mammal — B (from Agar). The figures are t3rawn to the same 

 scale 



archaic group of vertebrates which flourished along with Elasmobranchs 

 and Crossopterygians during the remote palaeozoic periods of geological 

 history. They are of great scientific interest from their mixture of 

 features that appear to have been relatively primitive characteristics of 

 the vertebrate phylum with others that foreshadow developments that 

 have taken place in the more highly evolved vertebrates. One of the 

 three genera — Lepidosiren — possesses another feature that renders it of 

 much scientific importance, namely that its nuclei and their constituent 

 chromosomes are of a size unequalled in any other vertebrate, indeed so 

 far as is known in any other animal (Fig. 159). This renders it peculiarly 

 adapted for the investigation of the minute details of nuclear structure 

 and one of the most important studies of these details in the repro- 



