206 ZOOLOGY SECT. 



but the proximal end of the ventral aorta is dilated to form a bulbus 

 aortce (b. a.), a structure which differs from a conus in being part 

 of the aorta, and not of the heart ; its walls do not contain striped 

 muscle, and are not rhythmically contractile. 



In accordance with the atrophy of the hyoid gill there is no 

 afferent branchial artery to that arch, but a hyoidean artery springs 

 from the ventral end of the first efferent branchial and passes to 

 the pseudobranch. The right branch of the caudal vein is continued 

 directly into the corresponding cardinal, the left breaks up in the 

 kidney, forming a renal-portal system. There are no lateral veins, 

 but the blood from the paired fins is returned to the cardinals. The 

 red blood-corpuscles are, as in other fishes, oval nucleated discs. 



Nervous System. The brain (Fig. 877) is very different from 

 that of Elasmobranchs, and is in many respects of a distinctly lower 

 type. The cerebellum (H.H.) is very large, and bent upon itself. 

 The optic lobes (M.H.) are also of great size, and on the ventral 

 surface are large bean-shaped lobi inferior es (U.L.). The dien- 

 cephalon is much reduced, and, indeed, is indicated dorsally only as 

 the place of origin of the pineal body (G. p.) : ventrally it is pro- 

 duced into the lobi inferiores with the infundibulum between them 

 giving attachment to the pituitary body (Hyp.). Hence, seen from 

 above, the small undivided prosencephalon (V.H.) comes imme- 

 diately in front of the mid-brain : it has a non-nervous roof (Pall.) 

 and its floor is raised into prominent corpora striata (BG., Bas. G.). 

 The olfactory bulbs, situated in close apposition with the prosen- 

 cephalon without intervening olfactory peduncles or olfactory tracts 

 such as are present in Scyllium (L.ol.), are nearly as large as the 

 corpora striata, and each contains a small cavity or rhinocoele in 

 communication with the undivided prosocoele. Three transverse 

 bands of fibres connect the right and left halves of the fore-brain, 

 an anterior commissure joining the corpora striata, a posterior 

 commissure situated just behind the origin of the pineal body, and 

 an inferior commissure in front of the infundibulum. The pineal 

 body (G.p.) is rounded and placed at the end of a hollow stalk : a 

 shorter offshoot of the roof of the diencephalon may perhaps 

 represent a rudimentary pineal eye. Behind the pituitary body 

 is a saccus vasculosus (s. v.). The optic nerves do not form a chiasma, 

 but simply cross one another, or decussate (Ch.), on leaving the 

 brain, the right nerve going to the left, and the left nerve to the 

 right eye, 



Sensory Organs. The most distinctive feature of the olfactory 

 sac is the possession of two small apertures, the anterior provided 

 with a valve. 



The eye (Fig. 878) has a very flat cornea (en.) with which the 

 globular lens (I.) is almost in contact, so that the aqueous chamber 

 of the eye is extremely small. Between the cartilaginous sclerotic 

 (scl.) and the vascular choroid (ch.) is a silvery layer or argentea 



