. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. VERTEBRAL COLUMN il vertebral column, and in these Fishes the anterior vertebras are fused into a single mass. In the caudal region of Amia the centra are mostly double, an archless pleura- or post-centrtim alternating with an inter- or pre-eentfy)n. A some- what similar condition is found in the Jurassic Eurycormus and other fossil Ganoids. As a rule Elasmobranchs and Ganoids possess a greater number of vertebree (in Alopecias vulpes there are 365) than Teleosts, in which we seldom meet with more than 70 : the Eel, ho
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. VERTEBRAL COLUMN il vertebral column, and in these Fishes the anterior vertebras are fused into a single mass. In the caudal region of Amia the centra are mostly double, an archless pleura- or post-centrtim alternating with an inter- or pre-eentfy)n. A some- what similar condition is found in the Jurassic Eurycormus and other fossil Ganoids. As a rule Elasmobranchs and Ganoids possess a greater number of vertebree (in Alopecias vulpes there are 365) than Teleosts, in which we seldom meet with more than 70 : the Eel, however, possesses more than 200. The caudal region of the vertebral column deserves particular attention in Fishes, and the condition of this region in Amphioxus, Cyclostomi and Dipnoi, may be taken as a starting-point. In these, the notochord extends straight backwards to the hinder end of the body and is surrounded quite symmetrically by the tail- fin, which is therefore spoken of as profocercal or diphyccrcal (Fig. 30). This condition is also met with in many Fishes of the. Fk4. 30.—Tail of ProtopUrus. Devonian strata as well as in young stages of Teleostei. In the latter, however, the ventral half of the tail-fin with its sup- porting skeleton (htemal arches and fin-rays) is, as a result of un- equal growth, more strongly developed than the dorsal, and the end of the vertebral column becomes bent upwards, thus giving rise to a heterocercal tail. This form of tail may be recognised externally, as in many Elasmobranchs, Ganoids, and numerous fossil Fishes; or may be masked by a more or less symmetrical tail-fin, as in Lepi- dosteus (Fig. 31), Amia, and more particularly in most Teleosts' { Salmo, Fig. 82), in which the heterocercal character is only visible internally. The posterior end of the vertebral column is then frequently represented by a rod-like urostylc, and in Teleosts one or more wedge-shaped hypttrcd hones (enlarged haemal arches) generally occur directly be
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectanatomycomparative
