86 MORPHOLOGICAL TYPES 



vena cava (right and left). The lateral abdominal veins unite 

 in the midventral line and so give rise to an anterior abdominal 

 vein, which runs into the right ductus Cuvieri close to its 

 connexion with the sinus venosus. 



The hindmost portions of the posterior cardinal veins 

 bring blood from the posterior region of the body to the 

 kidneys, and so form renal portal veins. Blood leaves the 

 kidneys by the left posterior cardinal and the inferior vena 

 cava. 



Urino-genital System. — The excretory system is similar 

 to that of Scyllium. The mesonephric kidneys are elongated, 

 and connect by means of Wolffian ducts with the cloaca. The 

 testis is connected with the kidney by vasa efferentia, and the 

 Wolffian duct functions as a vas deferens. In the female, the 

 eggs from the ovary are shed freely into the ccelom, and enter 

 the openings of the Mullerian ducts which lead them to the 

 cloaca. 



Nervous System. — The most remarkable feature of the 

 nervous system is the formation of cerebral hemispheres in 

 the telencephalon (end-brain). They are hollow outgrowths 

 from the diencephalon projecting forwards side by side. A 

 transverse section in the region of the brain of Ceratodus 

 therefore would show a pair of cavities, not a single cavity as 

 in lower forms. The cavities of the cerebral hemispheres 

 are the so-called first and second ventricles of the brain ; they 

 communicate with the cavity of the rest of the forebrain (third 

 ventricle) through the foramina of Monro. This is the first 

 appearance in the vertebrate series of the organs which mean 

 so much in the supremacy of man over other animals. In 

 Ceratodus, the roof of the cerebral hemispheres is still 

 membranous. The cerebellum is small. 



As regards the sense-organs, the eyes and ears present no 

 striking features. It must be remembered that the nasal 

 sacs each have two openings. The lateral line is somewhat 

 degenerate, and in some regions may consist of a groove 

 instead of a canal. 



