572 The Animal Kingdom 



Respiration is made possible by the seven pairs of gill pouches 

 which are well provided with gill filaments. Internally these pouches 

 are connected to the buccal cavity by means of the respiratory tube. 

 Water, however, does not pass through the mouth and over the gills 

 as is typical of most fish; instead, the water passes in and out of the 

 gill slits. Such an arrangement is imperative in an animal like the 

 lamprey which remains attached for periods of time to its prey. 



At the posterior end of the branchial basket is the pericardial sac 

 which contains the heart. There are but two chambers to the heart: 

 the ventricle and auricle. There is a series of veins and arteries and 

 lymphatic sinuses that distribute the blood throughout the body. 



NASAL OPENING. 

 PITUITARY SAC . 



uiierirc renpuACiiK ^—NEBVE CORD -NOTOCHORD 



^FACTORY BRAIN ^MUSCLES ESOPHAGUS /^pQ^^^^p^^^ ^ 



Fig. 191. — Sagittal section of ttie anterior end of the 

 adult sea lamprey, Petromyzon. 



The brain is relatively small and contains a pair of olfactory lobes, 

 small cerebral hemispheres, a diencephalon, a pair of large optic lobes, a 

 small cerebellum, and a well-developed medulla. Ventral to the dien- 

 cephalon is the infundibulmn and dorsally the pineal body. The spinal 

 cord is quite flat, and there are no sympathetic ganglia. 



The sense organs include the olfactory sac, the paired eyes, the 

 median pineal eye, and the auditory capsule. Taste buds are present on 

 the walls of the pharynx. A tube, the pituitary sac, arises from the 

 olfactory sac, and in the region of the infundibulum forms the pituitary 

 body. 



The kidneys are on the dorsal part of the body cavity and are con- 

 nected by the urinary duct to the urogenital sinus. This in turn opens to 

 the outside through the urogenital opening on the papilla. A single 

 gonad is present and fills most of the coelom. The gametes pass out 

 into the body cavity and go through two genital pores into the uro- 



