. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. CIO •« ANATOMY OF VERTEBRATES. from ramifications of a branch of the mesenteric artery, and the blood is returned to the mesenteric vein. Hunter's preparations of the embryo Carcharias (No. 1061), Scyllium (No. 3250), Spinax (No. .3255), and Alopias (No. 3261) ', demonstrate another foetal j)ecullarlty which later researches ^ have shown to be pro- bably common to all Plagiostomes, viz. the external fringe of filaments developed from the branchial surfaces, b: a tuft extends out of each aperture, and even from the spirac
. On the anatomy of vertebrates. Vertebrates; Anatomy, Comparative; 1866. CIO •« ANATOMY OF VERTEBRATES. from ramifications of a branch of the mesenteric artery, and the blood is returned to the mesenteric vein. Hunter's preparations of the embryo Carcharias (No. 1061), Scyllium (No. 3250), Spinax (No. .3255), and Alopias (No. 3261) ', demonstrate another foetal j)ecullarlty which later researches ^ have shown to be pro- bably common to all Plagiostomes, viz. the external fringe of filaments developed from the branchial surfaces, b: a tuft extends out of each aperture, and even from the spiracula, a, in the genera with those accessory openings. Each filament contains a single ca- pillary loop :' they disapjiear early, being removed by absorption. The last remnants may be seen in the preparation of the fcetal Saw- fish {Pristis, No. 3263),'' which is eight inches in length, including the saw, and has the duct of the external vitellicle attached. In the oviparous Sharks, the branchial filaments '*-*' react on the streams of water admitted into the egg by the apertures, fig. 426, c. In the ovo- viviparous Sharks the size and position of the cloacal apertures of the uteri would seem adapted to allow free ingress of sea-water; so that, whilst the vitellicle, ib. i, administers to the nutri- ment of the embryo, a, the external branchia; may perform the respiratoiy function. In the smooth Emissole {Mustclus levis), vascular cotyledons are developed from the vitelline (omphalo-mesen- teric) capillaries, which are firmly connected to the viterine cotyledons ; so that here the vitel- licle, like a true placenta, may perform both the nutrient and respiratory functions : the external branchiaj disappear some time before the exclusion of the embryo and the aljsorption of the yolk. In the Lcpidosircn annectms^ three small external branchial filaments project from the single opercular aperture on each side, and are long retained. Some of the Plagiostomons Fishes are oviparous,
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