CYCLOSTOMATA 417 



The skeletal system is cartilaginous, developed around a nonseg- 

 rnented notochord along each side of which are paired cartilages 

 called neural arches. At the anterior end is a skull whose floor and 

 sides are cartilaginous, while the roof is membranous, except for a 

 transverse bar. There are two auditory capsules near the posterior 

 part of the skull. The buccal funnel is supported by the annular 

 cartilage already mentioned and three sets of labial cartilages. The 

 branchial area is supported by the cartilaginous hranchial basket 

 which is composed of a i)air each of dorsal and ventral longitudinal 

 bars, two pairs of sinuous, lateral bars, and nine much-curved, dorso- 

 ventral bars. The anterior one of these is not in contact with a gill 

 aperture. The cartilaginous pericardium joins the branchial basket 

 at the posterior end. 



The digestive system is not very highly developed because the 

 adult lives entirely on blood and lymph of other fish, obtained by 

 rasping a hole through the body wall and sucking it out. They 

 take a meal about once in three or four weeks. The blood is passed 

 from the mouth down the esophagus which continues into the in- 

 testine at the level of the posterior end of the branchial region. 

 The intestine is slender and almost straight, but it has a slight 

 internal fold which extends spirally through its length. This is 

 called a typhlosole or spiral valve, and it tends to increase the absorp- 

 tive surface. The intestine ends posteriorly at the anus. The liver 

 is found in the anterior part of the body cavity. 



The circulatory system consists of a heart with two principal 

 chambers, arteries, capillaries, veins, and lymphatic spaces. The 

 posterior and anterior cardinal veins located just lateral to the lower 

 side of the notochord collect blood from the body wall and head 

 region, and empty it into the common cardinal vein which extends 

 ventrally to the sinus venosus. The sinus venosus receives also the 

 single inferior jugular and the hepatic vein from the ventral region. 

 The blood then passes through the sinuauricular valve to the single 

 auricle, thence by the auriculoventricular aperture to the single ven- 

 tricle, thence through hulhus arteriosus to the ventral aorta. Six 

 pairs of afferent branchial arteries carry the blood to the gills where 

 capillaries supply the gill lamellae. The efferent branchial arteries 

 collect this blood, carry it dorsally to join the dorsal aorta which is 

 made up by their convergence. A carotid branch of this artery 

 supplies the brain region, and the main aorta passes posteriorly. 



