(Aves), and Mammals (Mammalia). So far there is perfect unanimity; but when it is inquired into what larger sections the Vertebrata may be divided there is much difference of opinion. Here, the divisions proposed by Professor Huxley will be adopted; but it is necessary that those employed by other writers should be mentioned and explained.

One of the commonest methods of classifying the Vertebrata is to divide them into the two primary sections of the Branchiata and Abranchiata. Of these, the Branchiate section includes the Fishes and Amphibians, and is characterised by the fact that the animal is always provided at some period of its life with branchia or gills. The Abranchiate section includes the Reptiles, Birds, and Mammals, and is characterised by the fact that the animal is never provided at any time of its life with gills. Additional characters of the Branchiate Vertebrates are, that the embryo is not furnished with the structures known as the amnion and allantois. Hence the Branchiate Vertebrates are often spoken of as the Anamniota and as the Anallantoidea. In the Abranchiate Vertebrates, on the other hand, the embryo is always provided with an amnion and allantois, and hence this section is spoken of as the Amniota or as the Allantoidea.*

By Professor Owen the Vertebrata are divided into the two primary sections of the Hamatocrya and the Hæmatotherma, the characters of the blood being taken as the distinctive character. The Hæmatocrya or Cold-blooded Vertebrates comprise the Fishes, Amphibia, and Reptiles, and are characterised by their cold blood and imperfect circulation. The Hæmatotherma or Warm-blooded Vertebrates comprise the Birds and the Mammals, and are characterised by their hot blood, four-chambered heart, and complete separation of the pulmonary and systemic circulations. The chief objection to this division lies in the separation which is effected between

The amnion (fig. 222, C) is a membranous sac, containing a fluid-the liquor amnii-and completely enveloping the embryo. It constitutes one of the so-called "foetal membranes," and is thrown off at birth. The allantois (fig. 222, C) is an embryonic structure, which is developed out of the middle or 66 vascular" layer of the germinal membrane. It appears at first as a solid, pear-shaped, cellular mass, arising from the under part of the body of the embryo. In the process of development, the allantois increases largely in size, and becomes converted into a vesicle which envelops the embryo in part or wholly. It is abundantly supplied with blood, and is the organ whereby the blood of the foetus is aerated. The part of the allantois which is external to the body of the embryo is cast off at birth; but the portion which is within the body is retained, and is converted into the urinary bladder.

the Reptiles and the Birds, two classes which are certainly very nearly allied to one another.

By Professor Huxley the Vertebrata are divided into the following three primary sections:

I. ICHTHYOPSIDA. This section comprises the Fishes and the Amphibians, and is characterised by the presence at some period of life of gills or branchiæ, the absence of an amnion, the absence or rudimentary condition of the allantois, and the possession of nucleated red blood-corpuscles.

II. SAUROPSIDA.-This section comprises the Birds and the Reptiles, and is characterised by the constant absence of gills, the possession of an amnion and allantois, the articulation of the skull with the vertebral column by a single occipital condyle; the composition of each ramus of the lower jaw of several pieces, and the articulation of the lower jaw with the skull by the intervention of an "os quadratum;" and, lastly, the possession of nucleated red blood-corpuscles.

III. MAMMALIA.-This section includes the single class of the Mammals, and agrees with the preceding in never possessing gills, and in having an amnion and allantois. The Mammalia, however, differ from the Sauropsida in the fact that the skull articulates with the vertebral column by two occipital condyles; each ramus of the lower jaw is simple, composed of a single piece, and the lower jaw is united with the temporal (squamosal) element of the skull, and is not articulated to a quadrate bone. There are special glands - the mammary glands- for the nourishment of the young for a longer or shorter period after birth, and the red blood-corpuscles are non-nucleated.

I.

LITERATURE.

"Comparative Anatomy and Physiology of Vertebrates."
1866-68.

Owen.

2. "Manual of the Anatomy of Vertebrated Animals." Huxley. 1872. 3. "Principles of Comparative Physiology." W. B. Carpenter.

4. "Forms of Animal Life." Rolleston. 1870.

5

"General Outline of the Organisation of the Animal Kingdom." Rymer Jones.

1871.

6. "Handbook of Zoology." Van der Hoeven. Trans. by Dr W.

Clarke. 1856-58.

7. "Manual of Comparative Anatomy." Blumenbach. Trans. by

Laurence. 1827.

8. "Comparative Anatomy of Vertebrate Animals." Wagner. Trans.

9.

by Tulk.

1845.

"Grundriss der Vergleichenden Anatomie." Gegenbaur. 1874. 15. "Le Règne Animal distribué d'après son organisation." Cuvier. II. "Leçons d'anatomie comparée.' Cuvier.

66

12. System der Vergleichenden Anatomie." Meckel. 1821-33. 13. "Leçons sur la physiologie et l'anatomie comparée de l'homme et des animaux." Milne-Edwards. 1857-72.

14.

15.

"Lectures on the Elements of Comparative Anatomy." (On the Classification of Animals and on the Vertebrate Skull.) Huxley. 1864.

"Discourse on the Nature of Limbs." Owen.

1849.

16. "On the Shoulder-Girdle and Sternum." Parker. Ray Society.'

66

1868.

17. 'Lectures on Histology." Quekett. 1852-54.

18. "The Microscope and its Revelations." Carpenter.

19. "How to work with the Microscope." Beale.

20.

"Lehrbuch der Histologie des Menschen und der Thiere." Leydig. 1857.

21. Embryology, with the Physiology of Generation." Müller. Trans. by Baly. 1848.

22.

23.

"Elements of Embryology." Foster and Balfour. 1876. "The Geographical Distribution of Animals." Alfred Wallace. 1876. 24. "Recherches sur les ossemens fossiles." Cuvier. 1834-36. 25. "Manual of Paleontology." Owen. 1861.

26. "Traité de Paléontologie.'

27.

66

Pictet. 1853.

Hæckel.

"Cours élémentaire de Paléontologie." D'Orbigny. 1849. 28. "Manual of Paleontology." Nicholson. 1872. 29. "Generelle Morphologie der Organismen." 30. System der thierischen Morphologie.' Victor Carus. Entwickelungsgeschichte der Thiere." Von Baer. "The Morphology of the Skull." Parker & Bettany. "Lessons in Elementary Anatomy." St George Mivart. 1873.

31.

32.

33.

1866.

1853.

1837.

1877.

DIVISION I-ICHTHYOPSIDA.

CHAPTER LII.

CLASS I-PISCES.

THE first class of the Vertebrata is that of the Fishes (Pisces), which may be broadly defined as including Vertebrate animals which are provided with gills throughout the whole of life; the heart, when present, consists (except in Dipnot) of a single auricle and a single ventricle; the blood is cold; the limbs, when present, are in the form of fins, or expansions of the integument; and there is neither an amnion nor allantois in the embryo, unless the latter is represented by the urinary bladder.

In form, Fishes are adapted for rapid locomotion in water, the shape of the body being such as to give rise to the least possible friction in swimming.

To this end also, as well as for purposes of defence, the body is usually enveloped with a coating of scales developed in the inferior or dermal layer of the skin; whereas the epidermis is represented only by the slimy mucus covering the exterior of the animal. The more important modifications in the form of these dermal scales are as follows: I. Cycloid scales (fig. 230, a), consisting of thin,

e

flexible, horny or bony scales, Fig. 230.-Scales of different fishes.

circular or elliptical in shape, and having a more or less completely smooth outline. These are the scales which are charac

a

Cycloid scale (Pike); b Ctenoid scale (Perch); c Placoid scale (Thornback); d Placoid scale of Rhina; e Ganoid scales (Palæoniscus).

teristic of the most of the ordinary bony fishes. II. Ctenoid scales (fig. 230, b), also consisting of thin horny plates, but

having their posterior margins fringed with spines, or cut into comb-like projections. III. Ganoid scales, composed of an inferior layer of bone, covered by a superficial layer of hard polished enamel (the so-called "ganoine"). These scales (fig. 230, e) are usually much larger and thicker than the ordinary scales, and though they are often articulated to one another by special processes, they only rarely overlap. IV. Placoid scales, consisting of detached bony or dentinal grains, tubercles, or plates, of which the latter are not uncommonly armed with spines (fig. 230, c and d).

In most fishes there is also to be observed a line of peculiar scales, forming what is called the "lateral line." Each of the scales in this line is perforated by a tube leading down to a longitudinal canal which runs along the side of the body, and is connected with cavities in the head. The function of this singular system has been ordinarily believed to be that of secreting the mucus with which the surface of the body is covered; but this is certainly erroneous, and it seems to be more probably sensory in function, and to be connected with the sense of touch.

As regards their true osseous system or endoskeleton, Fishes vary very widely. In the Lancelet there can hardly be said to be any skeleton, the spinal cord being simply supported by the gelatinous notochord, which persists throughout life. In others the skeleton remains permanently cartilaginous; in others it is partially cartilaginous and partially ossified; and, lastly, in most modern fishes it is entirely ossified, or converted into bone.* Taking a bony fish (fig. 231) as in this respect a typical example of the class, the following are the chief points in the osteology of a fish which require notice :—

The vertebral column in a bony fish consists of vertebræ, which are hollow at both ends, or biconcave, and are technically said to be "amphicœlous." The cup-like margins of the vertebral bodies are united by ligaments, and the cavities formed between contiguous vertebræ are filled with the gelatinous remains of the notochord. This elastic gelatinous substance acts as a kind of ball-and-socket joint between the bodies of the vertebræ, thus giving the whole spine the extreme mobility which is requisite for animals living in a watery medium. The ossification of the vertebræ is often much more imperfect than the above, but in no case except that of the Bony Pike (Lepidesteus) is ossification carried to a greater extent than this.

In

* The so-called "bone" of the skeleton of Fishes is only occasionally true osseous tissue. In a great many instances it is a homogeneous or tubu lar, bone-like substance, or it may resemble genuine dentine.