Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXIII The Tail

An extension of the body behind the anus, containing all the chief tissues of the body, is a structure characteristic of chordate animals. Its original function was to assist the animal in swim- ming, for it contains myotomes and a portion of the notochord, and so is able to take part in the undulatory movements from side to side which propel the animal forwards through the water. The area of the tail is commonly increased by the formation of a fin in the middle line, in the lower chor dates. In Amphioxus, the fin is not very large, but it extends sym- metrically from the middorsal and midventral lines of the tail, and tapers to a point behind. This primitive type of tail is called diphycercal. It is present also in the Cyclostomes, where it is supported by cartilaginous radials, and in early stages of development of other forms.

In Selachians the tail is asymmetrical, for the vertebral column is bent slightly dorsally, and the dorsal (epichordal) lobe of the caudal fin is reduced while the ventral (hypochordal) lobe is increased. The ventral lobe is supported by the elongated haemal arches of the vertebral column, known as the hypurals, and not by separate radials. This type of fin is called heterocercal. In addition to the Selachii, it is present in the sturgeon, the Osteolepidoti and the fossil Dipnoi. Since the axis of the tail in such forms is bent up, in swimming, the head of the fish tends to be turned down, as when the fish noses along the bottom in search of its prey.

In the higher bony fish (Teleosts) the dorsal lobe of the caudal fin is further reduced and the ventral lobe enlarged, with the result that the tail presents an externally symmetrical (usually forked) appearance. Internally, however, the skeleton reveals the fact that this homocercal type of tail is derived from the heterocercal, and the axis can be seen to bend up at the tip. It is found also that during development, the homocercal tail passes through a heterocercal stage.

In other forms the tail tapers symmetrically to a point, and so comes to resemble the diphy cereal type. This secondarily simplified type of tail (shown by the eel, for example) is called gephyrocercal, and is the result of reduction from the hetero- cercal or homocercal condition. The tail-fin of Gadus is peculiar, for it is formed from the hind portions of the median dorsal and ventral fins. Such a tail is called pseudocaudal.

Fig. 165. — Skeleton of the tail of the salmon, showing the homocercal pattern of tail-fin characteristic of most Teleost fish.

Note the up -turned vertebral column, h, hypurals ; /, lepidotrichia ; v t vertebra.

In Ceratodus, the tail seems to be diphycercal (and there- fore primitive), because its ventral lobe is supported by separate radials, and not by hypurals. There is, however, doubt about this, because many of the fossil Dipnoi had heterocercal tails, and if it can be proved that Ceratodus is descended from them, the structure of its tail must be gephyrocercal.

In amphibia, the tail-fins are present in the larval stages, which live in water ; but they disappear when the animals come out on land, to grow again in some during the water sojourn of the breeding season. In the Anura (frogs and toads) the tail disappears altogether in the adult terrestrial form ; in the Urodeles (newts) it persists as a more or less tubular structure. In the Gymnophiona there is scarcely any tail at all, for the anus is almost at the hind extremity of the animal. In land-animals, the tail ceases to have the function which it exercised in the water, and it is often consequently much reduced. Instead of being a posterior prolongation of the body, it has the appearance of being merely an appendage, and it is of use to the animal in the maintenance of its balance, as a covering for the anus and genitalia, and in some cases as a fly- whisk.

Lizards have an interesting modification in that the vertebrae of the tail are cleft transversely, and it is at these points that the tail can be detached from the rest of the body. This faculty (autotomy) is of service to the animal in enabling it to escape from its enemies.

The primitive birds had long tails, with separate vertebrae, as is shown by Archaeopteryx. In living birds the caudal vertebrae are fused together to form the pygostyle, and the tail itself is much reduced. The so-called tail of birds consists of the tail-feathers.

In some arboreal animals, such as the chamaeleon and the American monkeys, the tail is prehensile and capable of grasping objects.

It is common to find that in those vertebrates which have returned to the water, the tail is well developed and expanded into fins. While superficially not unlike the tails and caudal fins of fish, they show in their structure fundamental differences. So in Ichthyosaurus, the vertebral column passes back into the ventral lobe of the fin ; in the whales the two lobes of the caudal fin are not dorsal and ventral but right and left, for the tail is expanded horizontally.

In the apes and man the external tail has disappeared altogether.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Cite this page: Hill, M.A. (2024, April 25) Embryology Book - Vertebrate Zoology (1928) 23. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Zoology_(1928)_23

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