. A manual of zoology. Zoology. 504 CIIORDATA. loops of the gill circulation, ^\'hen these are tle\elopeil, atjcrcnt hrancliial arteries, gill capillaries, and fijcrcut arteries can lie recognized, the latter uniting to form the dorsal aorta and also gi\"ing off the arteries (carotids), wliich go to the head. The iiephridia are a pair of large reddish-brown organs hing outside the liody cavity to the right and left of the verteliral column, usually ex- tending from heart to anus. Their ducts empt)' Kehind the aitus or in the dorsal wall of the intestine and arc often provided with enlarge
. A manual of zoology. Zoology. 504 CIIORDATA. loops of the gill circulation, ^\'hen these are tle\elopeil, atjcrcnt hrancliial arteries, gill capillaries, and fijcrcut arteries can lie recognized, the latter uniting to form the dorsal aorta and also gi\"ing off the arteries (carotids), wliich go to the head. The iiephridia are a pair of large reddish-brown organs hing outside the liody cavity to the right and left of the verteliral column, usually ex- tending from heart to anus. Their ducts empt)' Kehind the aitus or in the dorsal wall of the intestine and arc often provided with enlargements called, from their fuitctions, urinary bladders, although totally different. Fig. 554.—Heail of cmlir>-o telcost (diagram from GcgciibaurV a, auricle; ahr, ventral aorta witli arterial arches: <;i/, dorsal aorta; r, carotid; dc. Cu\ierian duct, formed by union of iugular and posterior cardinal veins; /;, nostril; .1, gill clefts; «', sinus veno- sus; ;', \'entricle. morphologically from the urinary bladder of the higher vertebrates. The gonatls, suspendeil liy mesorchia or mesovaria, are large aitil project into tlie body cavity. They are rarely unpaired. In the elasmobranchs and most ganoids their products pass out by the urogenital svstem (p. 487), in other forms by the pori alidomiitales or bv special ducts. A\"ith the exception of the Dipnoi and gaitoids which ha\-e ait unec|ual total cleavage, the eggs of fishes ha\-e a discoidal segmentation. Sub Class I. Elasmobranehii [Plagiostoiiii, Clioudroptervgii). The elasmobranchs, the shark-like fishes, are almost wholly marine' varying in length from a fool and a half to sixl}- feet, li\ing largely on other vertebrates, ami noted for their voracity. Sontetintes slender and cylintlrical (sharks, fig. 555), sometimes llaltened dorsoventrally (skates, hg. 550), they agree in that the head is prolonged into a snout, sup- ported by a cartilaginous prolongation of the cranium, the rostrum (hg. 540, R). The mouth is
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectzoology, bookyear1912
