Vertebrate Zoology G. R. De Beer (1928)

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Chapter I The Vertebrate Type as Contrasted with the Invertebrate

Although Vertebrate animals form the subject of this book, it must be said at once that, strictly, the term Chordate would be more correct as a title. The chorda dorsalis or notochord, from which the name is derived, made its appearance earlier in evolution than the vertebral column. There are therefore some animals which have a notochord but no vertebral column. On the other hand, all animals with a vertebral column also have a notochord at some time in their lives.

The term Vertebrate is used here partly because it is equivalent in importance to Invertebrate, and the most usual division of the animal kingdom lies between these two, and partly because attention is here paid particularly to the higher groups of " true " vertebrates. The lowly and peculiar Balanoglossids as well as the degenerate Ascidians will be left largely out of account, since they are not of much assistance in tracing the evolutionary history of the higher forms. Amphioxus as representative of the Cephalochordates, however, must be carefully considered on account of the help which it gives in interpreting and understanding various matters in higher forms.

The first necessity is to be clear as to what a chordate or vertebrate animal is, and how it differs in plan of structure from invertebrate animals (as typified, say, by Annelids or Arthropods).

A vertebrate is bilaterally symmetrical and moves typically in one direction with one side constantly presented upwards. A ccelom is present and the body, which is elongated from front to rear, is made up of a linear series of more or less similar blocks or segments. This repetition of parts or metameric segmentation affects tissues derived from all three of the primary layers from which the animal develops (see Chapter XI).

The gut on its way from mouth to anus is suspended in a fold of ccelomic epithelium forming a dorsal mesentery. The coelomic cavity can be separated into the following regions. The dorsal parts of the coelomic epithelium form the somites, which are segmentally arranged, and give rise to plates of muscle, or myotomes, one pair to each segment. The portion of ccelomic space associated with each myotome is a myocoel. The myoccel is bounded mesially by the myotome, and laterally by the cutis-layer of the coelomic epithelium. Slightly ventral to each myotome is a region of ccelomic epithelium which gives rise to excretory tubes (coelomoducts). This region is the nephrotome and its cavity the nephrocoel, also metamerically segmented. The ventral region of the ccelom is lined by epithelium (peritoneum) which forms the splanchnopleur where it is applied to the gut, and the somatopleur applied to the outer wall of the body. In this region the cavity is called the splanchnoccel.

The splanchnoccel is continuous from end to end of the animal, and uninterrupted by partitions or septa, except for that which separates an anterior pericardial from a posterior perivisceral space. This amounts to saying that the segmentation of the mesoderm does not persist in the ventral region. In higher forms much use is made of the free and uninterrupted space afforded by the splanchnoccel for the accommodation of longitudinal excretory and genital ducts, extensions of the liver and lungs, and coilings of the gut.

Fig. 1 (upper block). — Schematic view of a typical chordate animal in the region of the gill-slits.Part of the wall of the body is represented as removed in order to reveal the interior.

Fig. 2 (lower block). — Transverse section through a typical chordate animal (an embryo dogfish). c, splanchnocoel ; d, cutis-layer or dermatome ; da, dorsal aorta ; dn, dorsal nerve-root ; g, gut ; gs, gill-slit ; m, myotome ; mc, myocoel ; ms, mesentery ; n, notochord ; nc, nerve-cord ; nl, nephroccel ; nr, neural crest ; nt, nephrotome ; o, opening of gill-slit ; sc, sclerotome ; si, subintestinal blood-vessel ; so, somatopleur or body-wall ; sp, splanchnopleur or gut-wall ; v, blood-vessel running in the gill-arch between the gill-slits ; vn, ventral nerve-root.

It is customary to refer to the dorsal segmented regions of the mesoderm as vertebral plate, and to the ventral unsegmented portions as lateral plate.

The gut is primitively straight, leading from a mouth at the anterior end to an anus ; the latter is not at the extreme posterior end of the animal but some distance in front of it. Behind the anus is a well developed tail containing tissue derived from all three germ-layers. The possession of such a structure is one of the characteristics of the type as opposed to most invertebrates.

The blood flows in well-marked channels, and the direction of flow is forwards ventrally and backwards dorsally, which is the reverse of the invertebrate condition. The heart is ventral. Blood is led from the intestine to the liver by a hepatic portal vessel. A " portal " vessel is a vein which differs from others in that it not only starts from capillaries, but breaks up into capillaries again at the other end. The hepatic portal vein therefore runs from the capillaries of the intestine to those of the liver, and " carries " digested food- products thither. Ordinary veins do not break up into capillaries again, but connect with other veins and lead to the heart.

The nervous system is in the form of a hollow tube which runs all the way down the dorsal side of the animal, and contrasts sharply with the chief invertebrate type of two solid ventral nerve-cords, swelling out into ganglia in each segment. In vertebrates, the nerves are of two kinds, issuing from the nerve- tube by dorsal or by ventral roots.

The primitive respiratory system of vertebrates is equally distinctive, and consists of a number (usually five or six) of pairs of openings which lead from the front part of the gut to the outside — the gill-slits with their contained gills. These structures are among the most important, not only on account of their distinctiveness, but also because of the modifications which they undergo and the consequences which follow from their possession in evolution. The higher vertebrates breathe by means of lungs, which are sacs pushed out from the gut.

Running down the back of the animal, above the gut and beneath the nerve-tube, is a slender elastic rod which acts as a primitive skeleton. This is the notochord, which in higher forms is more or less obliterated and replaced by the backbone or vertebral column. The main skeleton of vertebrates is internal and not outside the body as in many invertebrates.

The higher forms have appendages, either fins or limbs, four in number arranged in two pairs. They are composed of tissue derived from several segments, not from one only as in invertebrates.

The differences and similarities between this fundamental plan and that of invertebrates may conveniently be set out in tabular form.

Vertebrates and most higher Invertebrates agree in being :

bilaterally symmetrical ;

coelomate ;

metamerically segmented. Vertebrates differ from Invertebrates in having :

a notochord ;

a dorsal and tubular nerve-cord ;

gill-slits ;

a postanal tail ;

a ventral heart through which blood flows forwards ;

main skeleton internal ;

appendages formed from several segments ;

a hepatic portal system ;

dorsal and ventral nerve-roots.

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 28) Embryology Book - Vertebrate Zoology (1928) 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Vertebrate_Zoology_(1928)_1

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