ferences; the Echinus Sphæra is remarkably in contrast. with the depressed form of the Echinocyamus pusillus. The soft and vermiform Holothurias are examples of other sub-types of the Echinoderms; still, a general plan can be traced in all.

In star-fishes and urchins, we find copious deposits of calcareous matter in the skin, in the form of distinct plates. M. Gaudry has very fully illustrated the general plan which regulates this part of their organization.* He has shewn that the protecting armour in all may be referred to three systems of parts-the endodermic or internal, the dermic or intermediate, and the epidermic or superficial. The internal system is absent in some. The dermic consists of four systems in parts-the ambulacral, so called from the locomotive function of the soft appendages which pass through them; the interambulacral, placed between the former series, and adding strength and solidity to the whole framework; the other two, ovarian, and anal or tergal plates, are respectively connected with the reproductive and digestive systems. The epidermic part of the armour comprehends all those appendages called spines, scales, tubercles, &c., which he shews to be formed after a common plan.

There are, moreover, traces of unity, when we examine the minute structure of the plates or of the superficial appendages. The microscope demonstrates that the hard matter consists of branches disposed vertically, and connected together by lateral branches, all of which are referable to a typical form.

But while the Radiate law generally regulates the external form (and the general arrangement of certain internal organs as well) we find that the number of the radii is also subject to law. However much a sea-star

* Annales des Sciences Naturelles, 1851.

seems to differ from a sea-urchin, the number five prevails in both. The question was long ago put by Sir Thomas Browne, "Why, among sea-stars, Nature chiefly delighteth in five points ?" and again, "By the same number (five) doth Nature divide the circle of the seastar, and in that order and number disposeth those elegant semicircles or dental sockets and eggs in the sea hedgehog." "Every plate of the sea-urchin," says Professor E. Forbes, "is built up of pentagonal particles. The skeletons of the digestive, the aquiferous, and tegumentary systems, equally present the quinary arrangement, and even the hard framework of the disc of every sucker is regulated by this mystic number."*

The same writer remarks, "When the parts of Echinoderms deviate from it (five) it is always either in consequence of the abortion of certain organs, or it is by a variation by representation, that is to say, by the assumption of the regnant number of another class. Thus do monstrous star-fishes and sea-urchins often appear quadrate, and have their parts fourfold, assuming the reigning number of Actinodermata, consistent with a law in which I put firm trust, that when parallel groups vary numerically by representation, they vary by interchange of their respective numbers."

Four is the number which generally prevails in the Acalephs or sea-jellies. In Cyanæa, for example, the stomach is usually subdivided by four; four æsophageal tubes are continued to their commencement, which is in the form of a quadrate mouth, the angles being prolonged into four tentacles. Sixteen canals radiate from the central cavities.†

In the charming Cydippe of our own seas, the same

*Forbes' British Star-fishes; Introduction.
↑ Owen's Lectures on Invertebrata, p. 165.

quaternary subdivision of the digestive system prevails. Moreover, the cirri by which it makes progression in the water, are arranged along eight equidistant bands.

The Actinæ or sea-anemones, not merely have some general resemblance to the well-known flower after which. they are named, but we find remarkable order as regards. number and relative position of organs, such as we have seen to prevail in plants. M. Hollard has shewn that the concentrical series of tentacula in the sea-anemone are subject to a law of alternation. This is well illustrated in the fullgrown Actinia senilis, the four concentric series of tentacles alternate with each other, and, as regards the numbers in each, the following is the formula:

10+10+20+40=80

FIG. 59.*

In some others the typical number is six or a multiple of six. Thus there are twelve tentacula in the first row in Actinia equina, six in Actinia pedunculata, and there are four rows in the first species, and five in the second.†

SECTION II.-ADAPTATION OF RADIATE TYPES TO THE

MODE OF LIFE.

Amid the general adherence to the Radiate type, we find modifications of parts in reference to locomotion, prehension, and retention of food, protection from external injury, and reproduction, all in evident accordance with the wants of the animals.

The simple Hydra of our fresh waters, consisting as it

FIG. 59. Plan of Sea-anemone, upper surface. The center circle represents the mouth. The smaller circles represent the tentacula in concentric and alternating series. + Annales des Sciences Naturelles, 1851.

does of little more than stomach, has, in the position, arrangement, and properties of its tentacula, admirable means of securing its prey. Its habits, and the adaptations of its organs, have been so often and so fully discussed elsewhere, that we need not dwell on the subject here. We merely allude to it in the outset on account of its relations to certain others of which it may be regarded as the type.

The little Hydra propagates both by a process of budding and by the formation of ova. The buds sprout out from the body of the parent, and passing through various stages, finally become detached and independent beings,

FIG. 60.t

each capable of producing others by the same process. But sometimes this mode of reproduction is so rapid, that each new Hydra-bud actually has buds of its own before it quits the parent stock. These buds, however, finally drop off and become independent, each forming a fresh colony.

The same mode of budding takes place in many others of the Hydroida, with this difference, that the buds usually remain attached to the stock or parent.

Zoophytes, &c.

See Trembley's Memoirs; Johnston's British FIG. 60. Hydra fusca propagating by buds. a, mouth; b, base or point of attachment. a to b, the original animal or stock from which the young or buds are formed;

c, point of origin of one of the buds.

But this building up of a tree-polyp could not proceed to any great extent if all polyps were entirely of the same soft texture as the Hydra. And here comes in a modification to which we owe many of those varied arborescent forms with which the ocean abounds, Long regarded as plants they are now well known to be compound Hydroida. The develop

ment of hard matter on the outside serves as a means of protection and support, in a medium liable as the sea is to such fluctuations in its condition. The soft material which pervades the centre of the hard covering is just a continuation of the digestive system of the polyps, each of which, protected in its little cell, captures food by means of its tentacula. The nourishment thus obtained contributes to the growth of the united colony, furnishing pabulum for the formation of new cells and new polyps. But there is another mode of propagation. There is a limit to the increase of the polyp-tree, and necessity for the establishment of new colonies at a distance from the parent. There appear at certain periods in the life of the Zoophyte, cells differing in form and size from those which protect the individual polyps.

FIG. 61.*

FIG. 61. Campanularia gelatinosa. A, fragment natural size; B, portion enlarged; b, young polype-bud; d, adult polype in its horny cell c; e, transformed branch with medusoid buds in different stages.