x HEART 449 



encloses the posterior hemibranch of one gill and the 

 anterior hemibranch of its immediate successor. 



The first pouch is situated between the hyoid and the 

 first branchial arch, and the hyoid thus bears a hemibranch 

 only. The first four branchial arches bear each a holo- 

 branch, and the fifth is without gill-filaments. There is 

 a vestigial hemibranch, or pseudobranch, on the anterior 

 (mandibular) wall of the spiracle. 



Now it is known that parts which have become useless 

 tend to disappear more or less completely (e.g., pineal body 

 of the frog p. 159, and certain of the gills in the crayfish 

 p. 375). In some cases, however, such vanishing parts 

 take on new relations with other organs and so once more 

 become useful in other ways, undergoing a change of function. 

 Thus in higher Vertebrates the spiracle is utilised in connec- 

 tion with the auditory organ, and instead of disappearing 

 entirely, as do the other gill-clefts, it gives rise to the 

 tympanic cavity and Eustachian tube (compare p. 45). 



Circulatory Organs. The heart is situated in the peri- 

 cardial cavity or anterior compartment of the ccelome 

 (p. 434) and is a large muscular organ composed of four 

 chambers. Posteriorly and dorsally is a small, thin- 

 walled sinus venosus (Figs. 117 and 119, s. v), opening in 

 front into a single, capacious, thin-walled auricle (au) ; 

 two auricles are present only in those Vertebrates which 

 possess lungs. The auricle communicates with a very 

 thick- walled ventricle (v), from which is given off in 

 front a tubular chamber, also with thick muscular walls, 

 the conus arteriosus (c. art). There are valves between 

 the sinus and the auricle, and between the auricle and 

 ventricle, and the conus contains two transverse rows, 

 each of three valves : all the valves are arranged so as 

 to allow of free passage of blood from sinus to auricle, 

 auricle to ventricle, and ventricle to conus, but to prevent 

 any flow in the opposite direction (compare pp. 79 and 87) . 



The conus gives off in front a single blood-vessel (v. ad] 

 having thick elastic walls composed, like other arteries, 

 of connective and elastic tissue and unstriped muscle. 

 This vessel, the ventral aorta, passes forwards beneath the 



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