COD. 19 



single row of valves. The aortic bulb differs from that of the 

 dog-fish in not being rythmieally contractile. The cardiac aorta 

 — the continuation upwards of the aortic bulb — divides into four 

 trunks the afferent branchial vessels, which carry the venous blood 

 to the gills ; they course round the outer side of the branchial 

 arches, and supply each filament. The now arterialised blood is 

 •collected into the efferent branchial vessels — one for each gill. 

 Each of these open into a trunk on either side — in fact, form it — 

 the right and left aortic arch. These two pass backwards, and 

 meet in the middle line beneath the vertebral column, and form 

 the dorsal aorta. 



The most anterior efferent branchial vessel gives off a large 

 carotid branch, which runs forward under the base of the skull, 

 and this is joined with its fellow by a transverse branch, so that 

 an entire arterial circle — the circulus cephalicus — is formed beneath 

 the base of the skull. The most anterior efferent branchial vessel 

 also gives off the hyoidean artery, which courses along the 

 hyoidean arch, and usually terminates by a branch in the cephalic 

 circle, and by another enters a rete mirabilia, which is situated in 

 the inner side of the hyomandibular bone, and sometimes simu- 

 lates a gill in appearance, and hence known as the pseudo- 

 branchia. The branches given off from the rete mirabilia join 

 again into the opthalmic artery, which, piercing the sclerotic, 

 breaks up into a second rete mirabilia — the so-called choroid 

 gland, before its final distribution. 



The efferent veins of the kidneys form on either side the 

 posterior venae cavae, which open separately into the sinus venosus. 

 Just before their entrance into the sinus venosus, the posterior 

 venae cavae receive the anterior venae cavae, which return the blood 

 from the anterior part of the body. 



RESPIRATORY ORGANS. 



The branchiae are situated in two special chambers, placed 



on each side of the neck. They are carried upon the outer 



convex sides of the branchial arches, which are a series of bony 



arches, five in number, connected below with the hyoid arch, and 



above united with the base of the skull. The concave borders of 



the branchial arches are beset with a series of processes, which 



project inward toward the mouth, the function of which is to 



prevent the passage of any particles to the branchiae which might 



injure their delicate structure. The branchiae themselves have 



the form of a double series of vascular plates or lamellae. These 



lamellae are flat and pointed in shape, and are covered with a very 



vascular mucous membrane, in which the capillaries of the 



branchial vessels ramify. . The number of plates on one process has 

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