x HEART 449 



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 holobranch, and the fifth is without gill- 

 filaments. There is a vestigial hemibranch, or pseudobranch, on the 

 anterior wall of the spiracle. 



Now it is known that parts which have become useless tend to dis- 

 appear 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, how- 

 ever, such vanishing parts take ori new relations with other organs and 

 thus once more become useful in other ways, undergoing a change of 

 function. Thus in higher Vertebrates the spiracle is utilised in con- 

 nection 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 three 

 longitudinal rows of 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. ao\ 

 having thick elastic walls composed of connective and elastic 

 tissue and unstriped muscle. This vessel, the ventral aorta, 

 passes forwards beneath the gills, and gives off on either 

 side paired lateral branches, the afferent branchial arteries 



PR ACT. ZOOL. G G 



