nervous cord, ganglionated at intervals, proceeding along the body. Symmetry bilateral.

III. Mollusca. Body not jointed or ringed across : ner

vous cord encircling the alimentary canal, and rami

fying through the body. Symmetry bilateral or spiral.

IV. Radiata. Nervous system absent or reduced to a ring round the alimentary canal with few radiating threads. Symmetry radiate round an axis.

There are some forms of plants and animals so slightly adapted to special purposes of life, as far as we have yet discovered, that life may be thought to reside in them only in a general form; the type of which-if that can be called typical which seems rather to be marked by absence of all but elementary organization-may be called cellular or rudimentary. This group, however, is but provisional, and will probably be hereafter better divided according as the nutritive and reproductive systems are more surely analysed by the microscope.

Under the great types of vegetable and animal structure which have been mentioned, naturalists find it convenient to adopt many classes, orders, families, genera, species and varieties. These are not so settled in any case as to be altogether free from change by fresh inquiries and discoveries: they have been augmented and modified by the discoveries of palæontology; which have filled several void spaces in

the series of affinities, and illustrated modern types by ancient parallels.

The adherence of plants and animals to the several leading types is sufficient in most cases to leave no doubt of the place of each; the typical peculiarity influences all parts of their structure,-the leaves, flowers and fruit of a plant-the limbs, dermal covering, composition, colour and temperature of the blood in an animal. So that a single leaf will generally decide the place of a plant in the three great divisions-a single drop of circulating fluid, the eye, a bone, a shell or crust, that of an animal. As an exception among plants we may mention those of the family Smilaces which yield Sarsaparilla, whose leaves are veined with the net-work of Dicotyledones, but whose seeds are formed on the model of Monocotyledones. Among animals the Polyzoa or Bryozoa were long ranked as Radiate animals, and there is now a difference of opinion regarding the place of the conspicuous family of Echinodermata, which some place at the head of the Radiata, but others join with Annulose animals.

Each great type comprehends several considerable subdivisions, in each of which a reigning idea may often be traced in the structure, and exemplified in the function. These are again subdivided in a manner suggestive of other ideas. For example, Mam

malia are separated from all other animals by their mode of rearing their young.

Among Mammalia quadrupedal motion on land appears to be the reigning idea, which determines the complete type; but the Cetacea, destined for aquatic life, are bipedal, and have their two legs altered to perform the work of pectoral fins, and the tail expanded to a propulsive instrument. Thus ideas are expressed which seem borrowed from fishes, to which in general form suited for easy motion in water the Cetaceans also correspond. So in early geological times we find Ichthyosaurus assuming that sort of conformity to fish-structure and form, which belongs to what Mac Leay calls analogy, while the real affinity of the whale is to ordinary Mammalia, and the real affinity of the Ichthyosaurus is to ordinary Reptilia.

In a different manner the quadrupedal maınmal is modified for flight in the air. The Bat takes up the extended interdigital membrane, and the sternal keel, by analogy with birds-as in older periods of the world, the Pterodactyle spread its wings and worked its pectoral muscles under the same peculiarities1. Instances of this kind might be greatly multiplied, but these may be sufficient to shew that each great type or subtype of structure admits of variations of

1 See the evidence of the sternal keel of Pterodactylus in the fine collection of Green Sand Fossils in the Cambridge Museum.

high importance, by adopting means, if we may so speak, to express the ideas which are prevalent in another division. Though each great type seems to have in a general sense one main destiny marked out for it by structure, place of residence, and habits, yet each of the three higher types (Mollusca, Articulata, Vertebrata) admits of modifications to suit some of them for watery, and others for aërial life; some for swimming, others for flying, some for climbing, others for burrowing into earth or wood or mud or stone. Thus every thing that lives, is especially constructed for its life1, in conformity with general types which admit of much modification, to suit particular purposes.

ADAPTATIONS OF LIFE STRUCTURE.

Some idea may be formed of the rich variety of adaptations of animal structures to the conditions under which they are appointed to live by a mere enumeration of a few well-known cases, such as the following variations in respect of the powers of attachment or locomotion.

Life in water is maintained in objects which exhibit a vast diversity of forms and magnitudes, and

1 La moindre facette d'os, la moindre apophyse a un caractère determiné, relatif à la classe, à l'ordre, au genre et à l'espèce auxquels elle appartient.—Cuvier, Ossemens Fossiles, Disc. prel.

various contrivances are employed to adapt them to the specific gravity of the liquid. Among those not so adjusted are a large portion of the shell-covered Mollusca, which by reason of the weight of the shell are for the most part collected on the beds of lakes, rivers or the sea. Within reach of the tidal agitation, where the water is well charged with air, several races of Mollusca attach themselves-as the young oyster by its lower shell fixes itself permanently and furnishes support to others. Anomia is attached by a sort of plug, Pinna and the young Mytilus by a byssus spun through the agency of a muscular organ called the foot, which in all the races mentioned loses its usual function of locomotion.

The attachment of Actinia by its broad base, and of Patella by its circular mantle, is not of so permanent a character. Perhaps this is also the case with the suspensional ligament of Lingula and Terebratula. More curious examples are furnished by the cusps for adhesion which cover the arms of Cephalopoda1, both recent and fossil, and the sucking surfaces of Remora.

Flotation is accomplished in some of the marine races by very obvious contrivances. In the large Medusida whose figure is hemispherical, a steady position with the mouth downward is maintained by

1 The Polypus of Aristotle and Homer.