performing at the same time the office of absorbents and conveying the chyle to the great intestinal vein (g), from which proceed the singular and beautiful respiratory plexuses (h, h), which are submitted to the influence of the sea water by contact with the branchial trees.

The aerated blood is conveyed to a great mesenteric trunk (i, i), or branchial vein, from which it is transmitted to he parietes of the body, and returns by the cloaca to form the intestinal artery.

Hunter has figured certain glandular sacs opening into the stem of the hollow branchiæ, which may be regarded as a rudimental form of an excretory or renal system.

The chief divisions of the nervous system consist of the pharyngeal ring, which is closely applied against the inner side of the calcareous circle, and of the flattened chords which proceed along the groove or middle interspace in each of the pairs of longitudinal muscles, which traverse the interior of the integument of the animal through its entire

length. The integument is also acted upon by transverse fibres which run external to the longitudinal bands; and such is the irritability of this muscular system, that when the Holothuria is disturbed or captured it will sometimes eject its sand-laden intestine and most of the other viscera by the cloacal aperture, and very effectually unfit itself for anatomical investigations.

The generative organs constitute, as in other Echinoderms, a very considerable part of the abdominal viscera in the breeding season; but they present a more complicated form: they consist of a branched system of long and slender cæcal tubes (fig. 67. r), opening externally by a single common canal, whose orifice is near the mouth. The generative organ of the male Holothuria resembles that of the female in structure; but the sexes may be readily recognised at the breeding season by the different character of the contents of the tubes, which are white or colourless in the male, whilst the ova present a reddish or yellowish hue.

The generative organs of the Sipunculus are two straight, slender, unbranched, blind tubes, symmetrically disposed, and terminating each by a distinct orifice at the anterior third of the body.

Among the few observations which have hitherto been recorded of the development of the Echinoderms, are some which are of great interest.

According to M. Sars, the star-fish, immediately after exclusion from the egg, presents a depressed, round form, with four short clubshaped appendages at the anterior extremity; the young animal moves by vibratile cilia with the four arms in advance: at the end of twelve days the five rays begin to grow, and in eight days more the hollow feet appear. The swimming motions have now ceased altogether; the four original ciliated arms shrink, and in a month they have entirely disappeared, and the animal exchanges its binary for the radiated figure.

The ova of the Comatulæ escape from each receptacle, through a round aperture, about the month of July, adhering together in a roundish cluster of about one hundred. About the time of the dispersion of these ova, the minute Pentacrini appear, attached to the stems and branches of corallines, and occasionally to sea-weed. This is attached by a convex calcareous plate, from the centre of which arises the column composed of about twenty-four joints. The capital of the column or body bears five bifurcating arms, which are at first simple, but afterwards acquire the pinnæ, and subsequently the dorsal cirri. They further resemble small Comatulæ in having a separate mouth and lateral prominent vent. These small Pentacrini attain the height of about three-fourths of an inch.

The small Pentacrini entirely disappear in September, at which season the young Comatulæ make their appearance. It is the opinion of Mr. Thompson, the discoverer of the Pentacrinus Europæus, that this pedunculated star-fish is a transitional state of the young Comatula, an opinion which is adopted by Mr. Thompson, Mr. Ball, and Mr. Forbes, experienced naturalists, who have each obtained and compared the Pentacrini and young Comatulæ. The actual metamorphosis of the Pentacrinus into the Comatula has not yet been seen: we must suppose that it enters life at first in the active stage of a ciliated gemmule; that it next selects the appropriate situation for its sedentary pentacrinite stage of existence, and, finally dropping from the stalk, by an act of transverse fission, a second time assumes a free condition of existence under its mature form. Nor are these metamorphoses a whit more extraordinary than those of the gelatinous Medusa: nay, the parallel would be extremely close, since we saw that the Cyanea entered life as a ciliated locomotive infusory, then became a sedentary polype, supported on a central stem, which, finally, resolved itself into the freely swimming Acalephans by several transverse fissions.

Other highly interesting considerations arise out of the predominance of the Pentacrinite forms over the Asteria or Echini, in the limestones of the ancient transition epoch in Geology. As we advance in our survey of the organisation and metamorphoses of animals, we shall meet with many examples, in which the embryonic forms and conditions of structure of existing species have, at former periods, been persistent and common, and represented by mature and procreative species, sometimes upon a gigantic scale.

LECTURE XI.

ANELLATA.

a

IN both the Infusorial and Entozoic classes the body assumes more perfect linear and bilateral form as the species advance in the scale of organisation; and we have seen in the subjects of the preceding discourse, that even the typical radiated class of the zoophytic sub-kingdom conducts by the Holothurian and Sipuncular families to the vermiform type of the articulated sub-kingdom, in which the vegetative principle of development, by the frequent repetition of similar parts, is still conspicuously manifested, but exercises its

K

energies in a linear direction, and forms successive segments from before backwards. We find, in fact, at the lowest step of the great Homogangliate series of the Animal Kingdom an extensive group of vermiform animals, some of which very closely resemble the Trematode, and others the Nematoid, Entozoa, and all are devoid of jointed limbs but they possess a distinct circulating system of arteries and veins, and in almost all the species the blood is red. They have therefore been called "red-blooded worms," 66 vers à sang rouge," and "anellides," by the French naturalists; in Latin Anellata, from anellus, a little ring, because the entire body of these worms is made up of a succession of segments like little rings.

The mind is not easily liberated from the sway of opinions that have long been held as authoritative; although Cuvier seems to have been the first to detect the exaggerated importance of the zoological character derived by Aristotle from the colour of the blood, yet the judgment of the great modern reformer of zoology continued to be so far biassed by that character, that in his latest edition of the Règne Animal," he continued to place the Anellides, on account of the colour of their circulating fluid, at the head of the articulate series, above the Crustaceans, above the Arachnidans, above the Insects, whose transitory larval condition these apodal worms seem permanently to represent.

66

The body of an Anellide is always very long, soft, and subdivided into a number of segments, for the most part closely resembling or identical with each other. In many species the first segment is so slightly modified as scarcely to deserve the name of head; in others it is the seat of higher senses and more varied functions, and is at once recognisable as the cephalic segment.

In the lowest forms of the Anellata the locomotive instruments are suctorial discs, as in the Trematode worms; but the suckers are always two in number, and are terminal in position. The species

are confluent, and in almost all there exists at the base of each a long soft cylindrical appendage called the "cirrus (c)." The bristles in the setigerous Anellides are their chief organs of locomotion, and at the same time their weapons of attack and defence. They are generally sharp, or barbed, and hard enough to readily penetrate the soft bodies against which they strike.

The nervous system of the Anellides presents a marked advance beyond its condition in the white-blooded parasitic worms; it consists of a double median central chord or chain of small ganglions, extending from one end of the body to the other; the two chords diverge anteriorly to allow the passage of the oesophagus, and again unite above that tube to form a distinct, though small, bilobed cephalic ganglion.

Most of the Anellides are provided with ocelli, and in many of them the head supports soft cylindrical tentacules called "antennæ :" they are obviously organs of touch, but differ from the antennæ of insects in the absence of joints. In the first appearance of these not yet well understood organs of sensation, which form so remarkable and conspicuous a character, and so important an endowment of the higher articulate classes, we have again an interesting illustration of the principle of vegetative repetition; for every setigerous tubercle in the Anellides with cephalic antennæ, has a similar organ of sensation the distinction is merely local and nominal; the feelers on the first segment being called "antennæ;" those on the other segments "cirri."

The mouth is at the lower surface of the head, or at the anterior extremity of the body in the acephalous Anellides; in some species it is provided with a protractile proboscis, and with lateral jaws in the form of curved dentated horny plates; and the alimentary canal is generally straight, and in some species simple; in others, provided with a greater or less number of lateral cæcums. The anus is situated above, or at the posterior extremity of, the body, and the degree of redness of the circulating fluid varies considerably; in some species it is very pale: in one or two it even presents a greenish hue it circulates in a closed and very complicated system of vessels, of which the chief dorsal one is distinguished by its undulatory pulsations; and in some species the circulation is further aided by contractile sinuses, called hearts.

All Anellides have organs of respiration, adapted in a few species for extracting oxygen directly from the atmosphere; and in the rest of the class through the medium of water in these the gills are usually external, and vary considerably in form and position.

Such are the general anatomical characters of the class Anellata,