384 THE DOGFISH less, xxix 



enters the mouth and passes by the internal clefts into the 

 branchial pouches, and thence outwards by the external clefts, 

 a constant supply of oxygen being thus ensured. The gill- 

 pouches are developed as offshoots of the pharynx, and the 

 respiratory epithelium is therefore endodermal, not ecto- 

 dermal, as in the starfish, crayfish, and mussel. 



The organs of circulation attain a degree of specialisation 

 not met with in any of our former types. The heart is 

 situated in the pericardial cavity or anterior compartment of 

 the ccelome, and is a large muscular organ composed of four 

 chambers. Posteriorly is a small, thin-walled sinus venosus 

 (Figs. 103 and 104, s. v), opening in front into a capacious 

 thin-walled auricle (au) ; this 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. The 

 heart, alone among the involuntary muscles, is formed of 

 striped fibres. 



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 ventralaorta, 

 passes forwards beneath the gills, and gives off on each side 

 paired lateral branches, the afferent branchial arteries 

 {a. br. a). Each afferent artery passes to the corresponding 

 gill, and there branches out into smaller and smaller arteries, 

 which finally become microscopic, and open into a network 

 of delicate tubes called capillaries, with which the connec- 

 tive tissue of the branchial filaments is permeated. The 



