Liverpool have likewise made enquiries as to the facilities for exhibiting an hourly time-signal on the Tuskar Rock. These points, in the English and Irish Channels respectively, are both advantageous for such signals; but some special wire communications would in both cases be necessary, so that the first outlay might be rather large, otherwise there would be no practical difficulty.

We have now completed our account, which intentionally has been confined to the time-operations in connection with the Royal Observatory at Greenwich. In conclusion we may however remark, that the local observatories in Liverpool, Edinburgh, and Glasgow have also for many years done much to give authoritative Greenwich time in their respective surrounding districts. In Glasgow there is a most extensive system of controlled public clocks, and Edinburgh and Liverpool both possess time guns.

THE ECHINUS, OR SEA-URCHIN.

BY ST. GEORGE MIVART, F.R.S.
[PLATE LXV.]

THE

HE Englishman, unacquainted with natural history, who for the first time visits Marseilles will, when he wanders down to its busy port, most probably have his curiosity awakened by baskets full of dark, round, spiny bodies (disclosing deep yellow parts within), each about the size of an egg. These are the sea-urchins, sea-eggs, or echini, which are largely affected by the good folks of Marseilles, and constitute one of the many objects of their fish-market which interest, surprise, or disgust

the northerner on his first arrival on the Mediterranean shore.

A large specimen of echinus well indeed merits its name of sea-urchin, for externally it presents an amazing resemblance to a rolled-up hedgehog, or urchin, being covered over with spines which in size and general appearance are very singularly like those of the last-named animal.

The resemblance, however, between these two animals is of the most superficial character only, and two creatures more really distinct could hardly be selected from the whole animal kingdom.

The sea-urchin presents us with a singular mixture of great simplicity of structure united with very great complexity. It is indeed an animal formed on a very low type, which, while strictly preserving that low type, yet preserves it in a wonderfully ornate condition with a quite prodigious number of complications and adornments.

The creature, when deprived of its spines, presents the appearance of a spheroidal, melon-shaped body (the so-called shell), furnished with two poles (each being provided with an aperture), and with lines, like meridians, running from the vicinity of one pole nearly to the opposite one. The shell when thus stripped is seen to be formed of a multitude of parts, to be covered with small rounded prominences, or tubercles, and to be perforated by a vast number of minute holes, or foramina.

The whole structure will at first naturally be thought to be an external skeleton, like the true shell of a mussel,* or the hard investment of a lobster. It is, however, nothing of the kind, but is a truly internal skeleton, inasmuch as the soft substance of the animal (the perisoma) coats it externally as well as internally.

The numerous, distinct, calcified plates which form the shell are arrayed in a very definite order. Of the two poles the inferior one is termed the "oral pole," as it is there that the mouth is to be found. The superior one is called the "apical pole," and this is the situation of the anus. Around the anus are placed some small and more or less irregular plates, termed "anal plates." External to and immediately below the anal plates are ten large ones, each perforated, and forming together a single circular row. These ten plates are the "ocular" and "genital" plates, and they alternate with each other so that each ocular plate is between two genital plates, and each genital plate is between two ocular ones. The perforation in each genital plate is the external opening of a genital duct. The perforation in each ocular plate is for an eye-spot. Below this circle of ten plates is the great bulk of the shell, which is called the "corona," and consists of five vertical tracts beset with perforations and termed the ambulacra. Alternating with these are five other tracts of greater width, destitute of pores and termed the interambulacra. Each ambulacrum and each interambulacrum is narrower at its apical than at its oral end, and consists of two vertical series of plates. Each of these plates is pentagonal, while two angles of each pentagon are right angles. A zigzag suture joins together the two vertical series of ambulacral plates in each ambulacrum, and a similar suture unites together the two vertical series of interambulacral plates in each interambulacrum. On the other hand, a straight suture unites the flat margins of each series of ambulacral plates with the flat margins of the adjacent series of interambulacral plates. The plates which form the ambulacrum are seen, when closely examined, to consist each again of three pieces (pore-plates), which form between them the six pores, or foramina, of each ambulacral plate.

The interambulacral plates, on the other hand, are single, and without perforations.

There are never more than two vertical series of plates in each ambulacrum and in each interambulacrum. Therefore there are five pairs of rows of ambulacral plates and five interspaces occupied by five pairs of rows of interambulacral plates.

See the article on "The Anatomy of the River-Mussel," by Mr. J. C, Galton, POPULAR SCIENCE REVIEW for July 1870.

† See the article on the Lobster in the same work for October 1838.