CYCLOSTOMATA 



419 



animal is not attached to a host, water may be drawn through the 

 mouth, under the velum, through the respiratory tube, through the 

 paired gills and to the outside through the seven pairs of external 

 apertures. The blood in the gill capillaries is aerated from the 

 oxygen carried in the water as it passes over the gill lamellae. 

 While the lamprey is attached to a host fish, the water is drawn 

 into the respiratory tube through the gill slits and then discharged 

 through them. 



— OLFACTORY SAC 

 OLFACTORY LOBES 



PINEAL EYE 

 RIGHT CEREBRAL 

 HEMISPHERE 



OPTIC NERVE 



RIGHT GANGLION 

 HABENULAE 



DIENCEPHALON 

 PITUITARY POUCH 



OCULO-MOTOR N. 



MIDDLE CHOROID 

 PLEXUS 

 OPTIC LOBES 

 TRIGEMINUS N, 

 CEREBELLUM 

 FOURTH VENTRICLE 



AUDITORY SAC 

 AUDITORY NERVE 

 MEDULLA OBLONGATA 



NOTOCHORD 

 VAGUS N. 



Fig. 226. — Brain of lamprey. 



Lateral view ; dorsal view. 

 Evans.) 



(Drawn by Titus C. 



The nervous system shows the development of a small, primitive 

 brain, which possesses all five principal divisions of a vertebrate 

 brain. From anterior to posterior it is composed of olfactory lobes, 

 cerebral hemispheres closely fused to preceding, single dicncephalon 

 with its dorsal epiphysis, midbrain with a pair of optic lobes, insig- 

 nificant narrow bandlike cerebellum just behind the optic lobes, and 

 the medulla just posterior to it. This continues directly posteriorly 

 as the flattened spinal cord. The roof of the brain is rather mem- 

 branous, as it is not entirely closed over. The sense organs include 



