HEART. 



151 



rarely farther behind, as in Symbranchidce. It is enclosed in 

 a pericardium, generally entirely separated from the abdo- 

 miaal cavity by a diaphragma, which is, in fact, the an- 

 terior portion of the peritoneum, strengthened by aponeurotic 

 fibres. However, in some fishes there is a communication 

 between the pericardial and peritoneal sacs, viz. in tJhe 

 Chondropterygians and Acipenser, whilst in the Myxiaoids 

 the pericardial sac is merely a continuation of the peritoneum. 



The heart is, relatively to the size of the body, very small, 

 and consists of three divisions : the atrium, with a large sinus 

 venosus into which the veins enter; the ventricle; and a 

 conical hollow swelling at the 

 beginning of the arterial system, 

 the structure of which forms one 

 of the most important characters 

 used in the classification of fishes. 

 In all PalmiehtJiyes (Figs. 66 and 

 67) this swelling is still a division 

 of the pulsating heart, being pro- 

 vided with a thick muscular stra- 

 tum; it is not sepafeted from the 

 ventricle by two valves opposite 

 to each other, but its interior is 

 fitted with a plurality of valves, 

 arranged in transverse series more 

 or less numerous in the various 

 groups of Palmichthyes. Zepido- 

 siren and Protopterus offer an ex- 

 ample of a modification of this 

 valvular arrangement, their valves being longitudinal, each 

 valve in fact being formed by the confluence of several 

 smaller ones situated behiud one another. This Palaeichthyan 

 type is called conus arteriosus. 



In Cyclostomes and Teleosteans (Fig. 68) the enlargement 



Fig. 67.— Heart of Ceratodus. 



Atrium ; b, Conus arteriosus ; 

 d, Papillary valve within the 

 conus ; e-g, Transverse rows of 

 Ganoid valves ; h, i. Anterior 

 arcus aortas ; k, I, Posterior arcus 

 aortffi ; v, Ventricle. 



