together they appear. In this membrane resides the faculty of secreting the carbonate of lime for the formation of shell, and this, as fast as it is secreted, is deposited, layer after layer, round the edges of each plate, so that these are constantly enlarging during the active growth of the animal, till it has attained its full size. The spines are deposited in the same way, by the membrane which clothes them, and exhibit, in their structure, as seen in the microscope, the most exquisitely beautiful and regular arrangement of particles that can well be conceived. Under the ceaseless activity of the vital power, the deposition of shelly matter proceeds, and results in a structure whose exactness and beauty it would be vain to imitate.

The bony contents of the shell are not at all less elaborate; and when we consider the apparently low grade of the animal for whose use they are designed, and the simplicity of other parts of its organization, we cannot fail to be struck with wonder at the amount of skill and contrivance lavished on its dental apparatus. The prehension of food is certainly the first requirement of animal life, and consequently we find the organs connected with its mastication and digestion,-the mouth and stomach, those that are most prominently developed in the lower animals. Some of these appear to be mere stomachs, endowed with a capacious gullet, and a set of sucking lips. In others, the organs for cutting or tearing food, or for grinding it to jelly, are extremely powerful; and when their strength is compared with the weakness or inertia of their other organs, they give us that impression of disproportion which leads to the idea of monstrosity,

OF THE EGG URCHIN.

147

and which may, in some degree, account for the disgust or horror with which we view many of these creatures. Indeed, except in their minute size, they resemble in their characters many of the fabled monsters of antiquity, whose voracity was one of their most appalling qualities. Magnify any of the insect race, or of the worms to the size of elephants or serpents, and what portentous monsters they become ! Their mouths and jaws seem utterly disproportionate in strength and complexity to the rest of their structure, though not at all disproportionate to the office which these scavengers of creation discharge in the general scheme. But few animals can boast a dental apparatus equal in complexity to that of the Egg Urchin; a set of harder-pointed teeth; more grinding jaws, with a surface regularly "dressed," like that of a millstone; or stronger and more varied muscular bands, by which the motions of the whole structure are regulated. In an Urchin of the usual size this system of bony jaws and teeth forms a conical body, about an inch and a half long, placed with its pointed end toward the large aperture at the base of the shell, and extending backwards into the body of the animal. It is attached by strong muscles to five bony arches that surround the mouth of the shell, and several other sets of muscles serve to propel it for ward, to cause it to retreat, to move the mass from side to side, or to cause the jaws to act one on another, like pairs of millstones. The cone consists of five triangular pieces or jaws, hollowed out, with an opening down the centre in front; arched behind, and with the two sides flattened and finely grooved. In the hollow

of these jaws is placed a long moveable tooth, which plays up and down. When the cone is put together, the flat, dressed surfaces of the five jaws, which stand round in a circle, are brought into contact. All the food which is received at the mouth must pass between these surfaces; and as there are systems of muscles which enable them to play up and down and across, a more perfect mill for grinding down the food cannot well be conceived. We have not space more fully to describe it, but the excellent popular account given by Professor Jones, and the examination of a living specimen, will enable any one to understand the uses of the several parts of this singular mechanism.

*

Of the same class with the Sea Urchins and Starfishes, but exhibiting its characters in a weaker degree, and shewing in form and structure a tendency towards the Annelides, are the Holothuriado, or Sea Cucumbers, of which several species occasionally come up in the dredge. Their name, Sea Cucumbers, is very expressive of their form in a contracted state, when the body shrinks up into an oblong mass, slightly tapering to each end, and rough with wrinkles and with the rows of suckingfeet, which it has in common with the Urchins and Starfishes. In its texture it is tough and leathery, without calcareous plates. The absence of a shell, the presence of feathery tentacula about the mouth, and the shape of the body, are differences between these creatures and the Urchins; while the two latter circumstances, together with the mode of progression by alternate contractions and extensions of the body, connect

* "General Outline of the Animal Kingdom," p. 166, &c.

them with the Annelides. The general form of this family may be understood from the annexed figure, which represents Thyone papillosa, a species found on various parts of the coast. It differs from other animals of the family chiefly by having its sucking-feet scattered in an irregular manner over the whole surface, instead of being confined to five rows along the angles of the body. In other respects it resembles most of its

kindred. Its length is about three inches, but it can at pleasure extend and contract considerably. The colour is a dusky brownish-white, sometimes varied with spots. The tentacula, commonly whitish, are ten in number, pinnate, and capable of being much expanded, or wholly retracted within the orifice of the mouth. In captivity it is not always easy to persuade it to put forth these delicate organs to their full extent, but a

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DEEP SEA ZOOPHYTES.

bath of clean water will often put it into good-humour. We have already noticed the self-destruction of some of the Starfishes: the Sea Cucumbers have an equally singular habit of a similar kind. As their skin is too tough and strong to admit of voluntary dismemberment, they resort to the unique mode of vomiting up their intestines-in fact, the whole of their internal organs. Yet it does not appear that life is destroyed by this process. At least, it does not suddenly cease; and, according to the observations of Sir J. G. Dalyell, the lost parts are renewed, after months have elapsed, even in cases where the disemboweling process has been carried to an extreme point, leaving "the body an empty sac." Holothuriæ are often taken with their internal parts more or less deficient, yet apparently existing in health and vigour in such, probably, the lost parts are in process of restoration.

:

Many interesting members of the class of zoophytes, or compound polypes, are met with in dredging, a general account of whose classification has been given in a former chapter. For a more specific account of the deepsea species, I must refer to Dr. Johnstone's admirable work on the subject, and will here just notice some of the order ASTEROIDA, which I purposely omitted when speaking of the other two orders, HYDROIDA and HELIANTHOIDA. This group is distinguished from either of the others by a readily seen character, namely, the softer parts of the compound animal invest and enclose the harder parts, or skeleton. The coral of all this group is therefore internal. The precious coral of commerce is the skeleton of one of these zoophytes; and so is the Gorgonia flabellum, or Venus's Fan, a well-known West