According to Hertwig, the teeth should be regarded as part of the tegumentary system, as they really represent the scales of the skate, situated within the buccal cavity.

In man, the last molar, or wisdom tooth, is a rudimentary tooth. The small-sized shallow crown, the diminished number of tubercles, the fusion of the roots, the tardy appearance and occasional absence altogether, are all indications of a rudimentary condition.

2. The Skeleton. With few exceptions, the articular surfaces of the bodies of mammalian vertebrates are covered in youth with bony plates. These sometimes become very thick, and are called terminal epiphyses. In some mammals -the Sirenians, for instance the terminal epiphyses have disappeared. In man they still exist, but in an advanced stage of degeneration. In the lower vertebrates, such as the crocodile, many more ribs are functional than in man. In the crocodile all the ribs connected with the cervical vertebrae are functional, whereas in man they have degenerated. Of one entire section of the human vertebral column-the tail-so fully developed in the majority of other vertebrates, only a vestige now remains.

Other rudimentary skeletal pieces are the lesser horn of the hyoid bone, the stylo-hyoidean ligament, the coracoid process, and the interclavicular ligament.

3. The Muscular System.The cutaneous muscles, those of the shell of the ear, and those that move the tail, which in most mammals are well developed, are still present in man, but have degenerated.

Further, there is to be found in man the intra-acetabular part (the round ligament) of the deep flexor muscle of the toes which is functional. in some animals-in young ostriches, for instance. In the adult ostrich the intra-acetabular part is separated from the rest of the muscle, which is attached to the pelvis. Traces remain in the horse of a connection between the intra- and the extra-acetabular parts; the muscle itself is divided into two parts, the pectineal muscle in the thigh, and the deep flexor muscle of the toe situated in the leg. In the orang-outang, this degeneration has made further advances than in man, the intraacetabular part of the muscle having entirely disappeared.1

4. The Nervous System.-Here we find numerous signs of degeneration, of which the following are a few examples:

In the brain the pineal gland, the last remaining vestige of what was formerly a functional eye, is present.

In the spinal cord the filum terminale still exists. We know that the spinal cord in man does not retain its normal thickness to the extremity of

1 See Sutton, Ligaments, their nature and morphology. London,

the vertebral column but is arrested at the first lumbar vertebra. There a considerable number of special nerves leave it, forming a mass of branches like a horse's tail. Along the centre of these nerves, in the middle line, a slender filament represents the spinal cord to the extremity of the coccyx. This filament is the filum terminale, the spinal cord in a condition of degeneration.

5. The Digestive System.-The cæcum and its vermiform appendage, are well known to be organs which have degenerated.

6. The Vascular System.-In quadrupeds the intercostal veins are vertical, the blood consequently flowing against gravity. These veins contain valves which indirectly facilitate the upward and onward flow of the blood by preventing it from running back. Man, being a biped with a vertical thorax, is provided with intercostal veins that are almost horizontal. The ancestral valves being no longer indispensable are in a condition of degeneration.

7. Sense Organs. In the olfactory organ there remains a degenerate Jacobson's organ. In the organ of sight there is a third eyelid in a state of degeneration. In the organ of hearing there remains on the shell of the ear a kind of point (Darwin's point) which is the last remaining vestige of the ancestral elongated and pointed ear.

8. Genito-urinary System.-There is a whole series of rudimentary organs in the genito-urinary system of the higher animals. As is well known,

the Wolffian body plays a considerable part in the formation of the system. This body, the primitive kidney, loses its urinary function at a certain stage of embryonic development, and the permanent kidney which gradually develops alongside, assumes the urinary function. Later on, the Wolffian body assumes new functions, becoming an important part of the genital apparatus.

In this transformation partial degeneration occurs, resulting in such reduced structures as the epididymis, the organ of Rosenmüller, the vas aberrans, etc. (see fig. 57).

§ 2. Rudimentary organs in various groups.

1

1. Cælenterates.-The Coelenterates comprise three great groups. The Anthozoa, of which the coral is a type, the Hydrazoa, which include fresh water Hydra and the common jelly-fish of our seas, and the Ctenophora, of which the chief representative in our seas is Cydippe, a globular transparent animal frequently to be found floating in large numbers on the surface of the water.

The colonies of Anthozoa are usually composed of individuals all exactly alike. In some species, however, in the Pennatulida and the Alcyonaria for instance, there is a distinct differentiation amongst the numerous individuals composing the colony. Side by side with sexual individuals 1 See C. Vogt and Emile Yung, Traité d'anatomie comparée, vol. i.

provided with tentacles and the eight mesenteric folds, are other far simpler individuals: the zooïdes, the function of which is respiratory and of which the greater part of the organs have degenerated; the generative organs are lacking, the tentacles are very small, and the mesenteric folds only number two instead of eight. Degeneration, then, is exhibited side by side with specialization.

Among the Hydromeduse similar examples abound. It is known that the polyp-like or medusa-like forms of this group which may live independently, frequently associate themselves together to form colonies, sometimes predominantly polypoïd, sometimes completely medusoïd, and occasionally a mixture of the two.

In these cases a marked polymorphism is often apparent. The different individuals become adapted to definite functions, and the corresponding organs undergo special development; the other parts of the body having become either unnecessary or merely accessory, begin to degenerate and finally disappear. Thus we see in Hydroïd colonies, not only the hydra-like members, nutritive, fixed and sterile, and the medusa-like members which are reproductive and become free from the colony, but also certain individuals which are termed gonophores. These gonophores are really medusa-like members which have lost their independent movement, and have consequently more or less lost both their tentacles and their umbrella-like discs,