_a_, Atrium; _b_, Conus arteriosus; _v_, Ventricle; _h_, Branchial artery for 3d and 4th gill; _k_, for the second; _l_, for the first; _m_, branch for the opercular gill; _d_, Single valve at the base of the conus; _e-g_, Transverse rows of Ganoid valves.] CHAPTER X. ORGANS OF CIRCULATION. The _Blood-corpuscles_ of fishes are, with one exception, of an elliptic shape; this exception is _Petromyzon_, which possesses circular, flat, or slightly biconvex blood-corpuscles. They vary much in size; they are smallest in Teleosteans and Cyclostomes, those of _Acerina cernua_ measuring 1/2461 of an inch in their longitudinal, and 1/3000 in their transverse diameter. As far as it is known at present the _Salmonidæ_ have the largest blood-corpuscles among Teleosteans, those of the salmon measuring 1/1524 by 1/2460 in., approaching those of the Sturgeon. Those of the Chondropterygians are still larger; and finally, _Lepidosiren_ has blood-corpuscles not much smaller than those of Perennibranchiates, viz.--1/570 by 1/941 in. Branchiostoma is the only fish which does not possess red blood-corpuscles. [See G. Gulliver, “Proc. Zool. Soc.,” 1862, p. 91; and 1870, p. 844; and 1872, p. 833.] Fishes, in common with the other Vertebrates, are provided with a complete circulation for the body, with another equally complete for the organs of respiration, and with a particular abdominal circulation, terminating at the liver by means of the _vena portæ_; but their peculiar character consists in this, that the branchial circulation alone is provided at its base with a muscular apparatus or _heart_, corresponding to the right half of the heart of Mammalia and Birds. The _Heart_ is situated between the branchial and abdominal cavities, between the two halves of the scapulary arch, rarely farther behind, as in _Symbranchidæ_. It is enclosed in a _pericardium_, generally entirely separated from the abdominal cavity by a diaphragma, which is, in fact, the anterior portion of the peritoneum, strengthened by aponeurotic fibres. However, in some fishes there is a communication between the pericardial and peritoneal sacs, viz. in the Chondropterygians and Acipenser, whilst in the Myxinoids the pericardial sac is merely a continuation of the peritoneum. [Illustration: Fig. 67.--Heart of Ceratodus. _a_, Atrium; _b_, Conus arteriosus; _d_, Papillary valve within the conus; _e-g_, Transverse rows of Ganoid valves; _h_, _i_, Anterior arcus aortæ; _k_, _l_, Posterior arcus aortæ; _v_, Ventricle.]