VERTEBRATE ANIMALS 207 



are always developed in the embryo, together with their character- 

 istic skeleton and vascular supply. The slits disappear entirely in 

 the adult, with the exception of the one pair which takes part in the 

 formation of the ear passages. Their skeleton is modified to form 

 the hyoid apparatus, and the remains of their blood-vessels constitute 

 the bases of the three great arterial arches of the adult. 



We have dealt with the three main anatomical features of 

 the Chordata, i.e. the presence of a notochord, a dorsal hollow central 

 nervous system, and the possession of gill slits, characters which are 

 in themselves diagnostic of the group, but the members of the group 

 also possess a number of other points in common. 



Three pairs of sense organs are present, namely, the olfactory, 

 the optic and the auditory, which are lodged in protecting capsules 

 more or less closely connected with the skull. The essential parts 

 of these are always derived directly or indirectly from the ectoderm, 

 e.g. the retina of the eye is developed as an outgrowth of the brain 

 which is itself ectodermal. In the lower classes, i.e. Fish and 

 Amphibia, ten pairs of cranial nerves come off from the brain with 

 the same origin and similar general distribution, being obviously 

 homologous. The higher classes, i.e. Reptiles, Birds and Mammals, 

 have in addition to these nerves two more cranial nerves, making 

 twelve altogether. 



The alimentary canal is a long folded tube running from an 

 anterior ventral mouth to an anus that is posterior and ventral, 

 and it is differentiated into distinct regions. The portion of the gut 

 just beyond the stomach receives the ducts of a well-marked liver 

 and pancreas. The liver has not only an arterial blood supply, but 

 a special series of veins forming the hepatic portal system gathers up 

 the blood from the intestine and conveys it to the liver. 



The vascular system is composed of a closed series of vessels 

 carrying the blood to and from the central organ, the heart, which is 

 composed of chambers separated from one another by valves. The 

 blood is composed of an almost colourless fluid, the plasma, in which 

 float large numbers of corpuscles, some white, and others, far more 

 numerous, red, owing to the contained haemoglobin. 



The kidneys are compact bodies composed of a number of urinary 

 tubules aggregated together and possessing a common duct, the 

 ureter, for the conveyance of the urine to the exterior. In addition 

 to the glands connected with the gut and skin possessing ducts 

 for the removal of their secretion, there are also a number of 

 ductless glands whose products are passed directly into the blood 

 stream. 



The skin is composed of an outer epidermis of ectodermal origin 

 and an underlying dermis derived from the mesoderm, and except 



