fashion, and may be something like the original mollusc. Whence that original sprang is uncertain, but the common occurrence of the trochosphere larva and some of the characters of the primitive Gasteropods (Neomenia, Chatoderma, Chiton) suggest the origin of molluscs from some "worm" type or other. We can be sure of

this, however, that the series must have divided at a very early epoch into two sets, the sluggish, sedentary, headless bivalves on the one hand, and the more active and aggressive snails and cuttlefish on the other.

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Relation to Man.-Irresistibly we think first of oysters, which Huxley describes as gustatory flashes of summer lightning," and over which neolithic man smacked his lips. But many others, cuttlefish, ear-shells (Haliotis), mussels (Mytilus edulis), periwinkles (Littorina littorea), cockles (Cardium cordatum), etc. etc., are used as food, and many more as bait. In ancient days, as even now, the shells of many were used for ornaments, instruments, lamps, vessels, coins, etc.; the inner layer of the shell furnishes mother-of-pearl; concretions around irritating particles become pearls in the pearl-oyster (Margaritana) and in a few others; the Tyrian purple was a secretion of the whelk Purpura and the related Murex; and the attaching byssus threads of the large bivalve Pinna may be woven like silk.

On the other hand, a few cuttlefish are large enough to be somewhat dangerous; the bivalve Teredo boring into ship-bottoms and piers is a formidable pest, baulked, however, by the prevalent use of metal sheathing; the snails and slugs are even more voracious than the birds which decimate them.

Conchology was for a while a craze, rare shells have changed hands at the cost of hundreds of pounds, such is the human "mania of owning things." But the shells are often fascinating in their beauty, and poetic fancy has played lovingly with such as the Nautilus.

CHAPTER XVI

BACKBONED ANIMALS

1. Balanoglossus-2. Tunicates-3. The Lancelet-4. RoundMouths or Cyclostomata- 5. Fishes - 6. Amphibians 7. Reptiles-8. Birds-9. Mammals

ACCORDING to Aristotle, fishes and all higher animals were "bloodcontaining," and thus distinguished from the lower animals, which he regarded as "bloodless." He was mistaken as to the absence of blood in lower animals, for in most it is present, but the line which he drew between higher and lower animals has been recognised in all subsequent classifications. Fishes, amphibians, reptiles, birds, and mammals differ markedly from molluscs, insects, crustaceans, "worms," and yet simpler animals. The former are backboned (Vertebrate), the latter backboneless (Invertebrate).

It is necessary to make the contrast more precise. (a) Many Invertebrates have a well-developed nerve-cord, but this lies on the ventral surface of the body, and is connected anteriorly, by a ring round the gullet, with a dorsal brain in the head. In Vertebrates the whole of the central nervous system lies along the dorsal

FIG. 49.-Diagram of "Ideal Vertebrate," showing the segments of the body, the spinal cord, the notochord, the gill-clefts, the ventral heart. (After Haeckel.)

surface of the body, forming the brain and spinal cord. These arise by the infolding of a skin groove on the dorsal surface of the embryo. (6) Underneath the nerve-cord in the Vertebrate embryo

is a supporting rod or notochord. It arises along the roof of the food-canal, and serves as a supporting axis to the body. It persists in some of the lowest Vertebrates (e.g. the lancelet); it persists in part in some fishes; but in most Vertebrates it is replaced by a new growth-the backbone-which ensheaths and constricts it. (c) From the anterior region of the food-canal in fishes and tadpoles slits, bordered by gills, open to the exterior. Through the slits water flows, washing the outsides of blood-vessels and aërating the blood. These slits or clefts are represented in the young of all Vertebrate animals, but in reptiles, birds, and mammals they are transitory and never used. Amphibians are the highest animals in which they are used for breathing, and even then they may be entirely replaced by lungs in adult life. They are evident in tadpoles, they have disappeared in frogs. (d) Many an Invertebrate has a well-developed heart, but this always lies on the dorsal surface of the body, while that of fish or frog, bird or man, lies ventrally. (e) It is characteristic of the eye of backboned animals that the greater part of it arises as an outgrowth from the brain, while that of backboneless animals is directly derived from the skin. But this difference is less striking when we remember that it is from an infolding of skin that the brain of a backboned animal arises.

But while the characteristics of backboned animals can now be stated with a precision greater than that of sixty years ago, it is no longer possible to draw with a firm hand the dividing line between backboned and backboneless. Thus fishes are not the simplest Vertebrates; the lamprey and the glutinous hag belong to a more primitive type, and are called fishes only by courtesy; simpler still is the lancelet; the Tunicates hesitate on the border line, being tadpole-like in their youth, but mostly degenerate when adults; and the worm-like Balanoglossus is perhaps to be ranked as an incipient Vertebrate. The extension of knowledge and the application of evolutionary conceptions obliterate the ancient landmarks of more rigid but less natural classification.

1. Balanoglossus.-Balanoglossus is a worm - like animal, represented by some half-dozen species, which eat their way

FIG. 50.-Balanoglossus, showing proboscis, collar, and gill-slits.

through sandy mud off the coasts of the Channel Islands, Brittany, Chesapeake Bay, and other regions. Its body is ciliated and divided into distinct regions—a large "proboscis" in front of the mouth, a

firm collar behind the mouth, a part with numerous gill-slits behind the collar, and finally a soft coiled portion with the intestine and reproductive organs. The size varies from about an inch to 6 inches, the colours are bright, the odour is peculiar; the sexes are separate. But Balanoglossus is most remarkable in having a dorsal supporting rod (like a notochord) in the "proboscis" region, a dorsal nerve-cord running along the back and especially developed in the collar, and a series of gill-clefts on the anterior part of the food-canal. It is therefore difficult to exclude Balanoglossus from

FIG. 51.--Cephalodiscus, a single individual, isolated from a colony. It is much magnified. (From Chambers's Encyclop.; after Challenger Report, by M'Intosh and Harmer.)

the Vertebrate series, and it is likely that the same must be said of another strange animal, Cephalodiscus, discovered by the Challenger explorers.

2. Tunicates.-Hanging to the pennon-like seaweeds which fringe the rocky shore and are rarely uncovered by the tide, large sea-squirts sometimes live. They are shaped like double-mouthed wine-bags, 2 or 3 inches in length, and water is always being drawn in at one aperture and expelled at the other. Usually they live in clusters, and their life is very passive. We call them seasquirts because water may spout forth when we squeeze their

bodies, while the title Tunicate refers to a characteristic cloak or tunic which envelops the whole animal.

There is not much to suggest backbonedness about these Tunicates, and till 1866 no one dreamt that they could be included in the Vertebrate series. But then the Russian naturalist Kowalevsky discovered their life-history. The young forms are free-swimming creatures like miniature tadpoles, with a dorsal nerve-cord, a supporting rod in the tail region, gill-slits opening from the food canal, a little eye arising as an outgrowth of the brain, and a ventral heart.

There are only two or three genera of Tunicates, especially one called Appendicularia, in which these Vertebrate characteristics are retained throughout life. The others lose them more or less completely. The young Tunicates are active, perhaps too active, for a short time; then they settle down as if fatigued, fix themselves by their heads, absorb their tails, and become deformed. The nervous system is reduced to a single ganglion between the two apertures; the original gill-slits are replaced by a great number of a different character; the eye is lost. From the skin of the degenerate animal the external tunic is exuded. It is a cuticle, and consists, in part at least, of cellulose, the substance which forms the cell-walls of plants. Thus this characteristically vegetable substance occurs almost uniquely in the most passive part of a very passive animal. The sea-squirt's metamorphosis is one of the most signal instances of degeneration; the larva has a higher structure than the adult; the young Tunicate is a Vertebrate, the adult is a nondescript. We cannot tell how this fate has befallen the majority, nor why a few are free-swimmers, nor why Appendicularia retains throughout life the Vertebrate characteristics of its youth. Do the majority overexert themselves when they are "tadpoles," or are they constitutionally doomed to become sedentary?

Tunicates are hermaphrodite—a very rare condition among Vertebrates; some of them exhibit "alternation of generations," as the poet Chamisso first observed; asexual multiplication by budding is very common, and not only clusters but more or less intimate colonies are thus formed.

Tunicates live in all seas, mostly near the coast from low water to 20 fathoms, and usually fixed to stones and rocks, shells and seaweed. A few are free-swimming, such as the fire-flame (Pyrosoma), a unified colony of tubular form, sometimes 2 or 3 feet in length, and brilliantly phosphorescent. Very beautiful are the swimming chains of the genus Salpa, whose structure and life-history alike are complicated.

Tunicates feed on the animalcules borne in by the water currents, and some of them must feed well, so rapidly do they grow