"xm PHYLUM CHORDATA 197 



abdominal cavity is the air-bladder (a. bL), a thin-walled sac 

 serving as an organ of flotation. Anteriorly its ventral wall 

 presents a small aperture leading, by a short pneumatic duct 

 (pn.d.\ into the pharynx. 



Respiratory Organs. There are four pairs of gills each with 

 a double row of branchial filaments united proximally but having 

 their, distal ends free: interbranchial septa are practically obsolete 

 (see Fig. 726). The gills are borne on the first four branchial 

 arches, the fifth arch bearing no gill. On the inner surface of 

 the operculum is a comb-like body, the pseudo-branchia, formed 

 of a single row of branchial filaments, and representing the 

 vestigial gill (hemibranch) of the hyoid arch. 



Circulatory Organs. The heart (Fig. 817) consists of sinus 

 venosus, auricle (au.), and ventricle (v.). There is no conus 

 arteriosus, 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.i 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. 818) 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 corresponding 

 with them on the ventral surface are large bean-shaped lobi 

 inferiores ( IT. L.). The diencephalon is much reduced, and, indeed, 

 is indicated dorsally only as the place of origin *of the pineal 

 body (G.p.): ventrally it is produced into the lobi inferiores with 

 the infundibulum between them giving attachment to the 

 pituitary body (Hy. p.). Hence, seen from above, the small 

 undivided pi\>senceplialon (V.H.) comes immediately in front of 

 the mid-brain : it has a non-nervous roof or pallium (Pall.) and 

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

 The olfactory lobes (L. ol.) are nearly as large as the corpora striata, 

 and each contains a small cavity or rhinoccele in communication 

 with the undivided prosoccele. 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 (b.p.) 



