116 THE ANATOMY OF VERTEBEATED ANIMALS. 



higher Vertehrata. The heart is a simple tube, and the liver is 

 saccular. (Leptocardia. Haeckel). 



I. — Pharyngobranchii. 

 B. The notochord ends behind the pituitary fossa. A skull, brain, au- 

 ditory, and renal organs are developed. The heart is divided 

 into auricular and ventricular chambers. The liver has the 

 ordinary structure (Pachvcardia. Hck.). 



a. The nasal sac is single, and has a median external 

 aperture. Neither mandibles nor limb arches are deve- 

 loped {3Tono!'hina. Hck.). 



II. — Marsipohranchii. 

 b. There are two nasal sacs with separate apertures. Mandibles 

 and limb arches are developed. {Ampliir/iina. Hck.). 

 a. The nasal passages do not communicate with the cavity 

 of the mouth. There are no lungs, and the heart 

 has but one auricle, 

 a. The skull is devoid of membrane bones. 



in. — Elasmobrancli ii. 

 P. Membrane bones are developed in relation with 

 the skull. 



1. The optic nerves form a chiasma, and there 



are several rows of valves in the aortic bulb. 

 IV. Ganoidei. 



2. The optic nerves simply cross, and there is 



only one row of valves in the aortic bulb. 



V. — Teleostei. 

 h. The nasal passages communicate with the oral cavity. 

 There are lungs, and the heart has two auricles. 



VI.— i'zjonoi'. 



I. The Phaeyngobranchii.— This order contains but 

 one species of fish, the remarkable Lancelot, or Ampliioxus 

 lanceolatus, which lives in sand, at moderate depths in the 

 sea, in many parts of the world. It is a small semitrans- 

 parent creature, pointed at both ends, as its name implies, 

 and possessing no limbs, nor any hard epidermic or dermal 

 covering. 



The dorsal and caudal regions of the hody present a 

 low median fold of integument, which is the sole represen- 

 tative of the system of the median fins of other fishes. The 

 mouth (Fig. 28, A, a) is a proportionally large oval aper- 

 ture, which lies behind, as weU as below, the anterior ter- 

 mination of the body, and has its long axis directed longi- 

 tudinally. Its margins are produced into delicate ciliated 

 tentacles, supported by semi- cartilaginous filaments, which 



