212 ZOOLOGY sect. 



(jni. d.) into the oesophagus on the dorsal side somewhat to the 

 ricfht of the middle line. 



Respiratory Organs. — There are four pair 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. 771). The gills are borne on the first four branchial 

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

 the operculum is a comb-like body, the pseudo-hrcmchia, formed of 

 a single row of branchial filaments, and representing the vestigial 

 gill (hemibranch) of the hyoid arch. 



Circulatory Organs. — The heart (Fig. 865) consists of sinus 

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

 arteriosus, but the proximal end of the ventral aorta is dilated to 

 form a hulhus aortce (h. 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 hijoidcanarteri/ 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. 866) is very different from 

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

 type. The ce'rehcUum {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 (U.L.). The diencephalon is much reduced, and, indeed, 

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

 body {Li. 2').) : ventrally it is produced into the lobi inferiores with 

 the infundibulum between them giving attachment to the 

 pituitary body {Hyp.). Hence, seen from above, the small 

 undivided proseneephcdon ( V.H.) comes immediately in front of 

 the mid-brain : it has a non-nervous roof {Fall.) and its floor 

 is raised into prominent corpora striata {BG., Bas. G.). The 

 olfactory hidhs, situated in close apposition with the prosence- 

 phalon without intervening olfactory peduncles or olfactory tracts 

 such as are present m Scylliiim {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 

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

 an inferior commissure in front of the infundibulum. The pinecd 



