21 6 CHORD ATA 



Circulatory system consists of a dorsal vessel (paired and un- 

 paired dorsal aortae); a ventral vessel (subintestinal vein and ven- 

 tral aorta); and commissural, connecting, afferent and efferent 

 branchial arteries with intestinal capillaries. The circulation differs 

 from that in the Annulata, since the blood in the ventral vessel 

 travels forwards and the blood in the dorsal vessel travels backwards 

 which is the opposite from the condition in the Annelida. All the 

 intestinal blood passes through the liver before reaching the ventral 

 aorta. The hepatic portal system is characteristic of all verte- 

 brates. The blood is almost colorless with no leucocytes and but 

 few erythrocytes. 



Excretory System. — There are 90 pairs o( nephridia situated above 

 the pharynx. Columnar and excretory cells are situated on the 

 floor of the atrium. 



Reproductive System. — The sexes are separate, 26 pairs o{ gonadial 

 pouches opening into the atrium. The ripe germ cells burst from the 

 inner walls of the gonadial pouches and escape by way of the atrium 

 and atriopore to the external water where fertilization takes place. 



Nervous System. — The dorsal nerve tube has an axial cavity, the 

 neurocele, which is dilated anteriorly to form the cerebral ventricle. 

 The dorsal portion of this ventricle is dilated into a pointed pouch, 

 the median olfactory lobe, while in the ventral posterior portion there 

 is a depression probably corresponding to the infu7jdibulu7n of higher 

 forms. A large number of spinal nerves come from the spinal cord. 

 They arise alternately, in each segment, two dorsal nerves, sensory 

 and motor, supplying the skin and transverse muscles; and two 

 ventral nerves, purely motor, supplying the myotomes. 



Sense Organs. — The olfactory pit {hypophysis) is a ciliated depres- 

 sion opening externally on the left side of the snout. The median 

 cerebral " eye " has no lens and may not be sensitive to light. The 

 socalled gustatory groove on the roof of the buccal cavity may not 

 be an organ of taste. There are no equilibratory or auditory organs 

 known. 



Kconomic Importance. — Vast quantities of the Amphioxus are 

 used as food by the Orientals, particularly the Chinese. 



Theories of the Origin of Vertebrates 



Theories of the origin of vertebrates include the Amphioxus 

 theory, the Annelid theory, the Nemertean theory and the Arthropod 



