Actiniæ; in a third, Corallines, Annelids, Polyps, Rock-borers, Sabellæ, Serpulæ, Holothuriæ, Actiniæ, and so forth-classifying with care the occupants of each tank according to their various habits.

Temperature is a point of great importance. The mean temperature of the ocean is estimated to be about 56° Fahr., and this under ordinary circumstances does not vary more than about 12° throughout the different seasons of the year.

Many of the inhabitants of the sea are very sensitive to changes of temperature, and we find that a few degrees of variation will cause them rapidly to move their position, and seek some cooler or warmer spot, as the case may be. In the ocean it will be evident that the creatures have the power readily to effect this, under ordinary circumstances, by seeking deeper water, not liable to be affected by atmospheric influences, by partially, or entirely, burying themselves in the sand or shingle, or by shielding their bodies under the protecting shadow of the rocks, or growing vegetation. In arranging the rock-work in the interior of the aquarium, therefore, great care should be taken to keep these points in view, and to afford as much protection as possible to the creatures from the cooling influences of radiation on the one hand, and from the heat of the sun's rays upon the other. "From my own experience," says Mr. Warington, "I find that the range of temperature should not be below 50° Fahr., nor above 70°. Within these limits all appears to progress healthily; but beyond these points many of the creatures are rapidly affected.

* Warington, Ann. Nat. Hist. 1855.

MANAGEMENT OF LIGHT AND TEMPERATURE.

25

During the last long-continued severe winter it was found very difficult, in an ordinary sitting-room having a south aspect, and a good fire maintained throughout the day,-the tanks being also screened at night by a blind,—to prevent the powerful cooling effects from radiation on a clear frosty night; and on three several occasions, marking exactly the three severest frosts that we experienced during the winter, the thermometer immersed in an aquarium containing about thirty gallons of water fell as low as 45° Fahr. The Shrimp and Crab tribes, and the Crustaceans generally, are especially affected by these changes, and on each of the three occasions alluded to one or two individuals perished.”

Excess of heat, and also strong sunlight, are likewise to be as carefully guarded against, and the exposure of a small tank to the direct rays of the sun on a hot summer's day, will speedily result in the destruction of every animal in it.

Too much light has also the effect of rapidly causing the propagation of microscopic Infusoria of a green colour, such as Euglena and its congeners, which, under this influence, multiply so rapidly as to render the whole water turbid and opaque. These will sometimes subside to the lower part of the tank, as evening approaches, and disappear in the shingle bottom; but immediately the morning light shines strongly upon the aquarium, they will rise like a thin green cloud, and diffuse themselves throughout the water. Although this Infusorial growth is not unhealthy, yet it causes the aquarium to present a very unsightly appearance, and prevents all observation on

C

the habits of its inmates. The want of light, on the other hand, causes the rapid decay of the vegetation, and the products arising from this change are highly poisonous to animal life,-the whole contents of the aquarium becoming of a black colour, and very soon of an offensive odour.

All marine productions dislodged from considerable depths are liable to injury. Though casually obtained clean and entire, most of them are profusely invested with parasites, which, fatally and invisibly wounded, speedily corrupt the circumambient fluid in their decay. The water also sometimes contracts a noxious principle from causes eluding conjecture and observation, and it may be fit to warn the inexperienced naturalist against employing tall vessels with much empty space above the water on all occasions; for if the air vitiated by respiration, after escaping from the surface, be reabsorbed, it cannot be otherwise than deleterious. This is well illustrated by keeping fishes in deep vessels with a scanty proportion of water, to prevent their leaping over the side; they generally perish in a short time, and hence shallower vessels nearly full are always preferable.

It is sometimes advisable to suspend delicate or interesting zoophytes, more especially of the Tubularian or Sertularian races, by means of silken threads attached to pieces of cork, which may be allowed to float upon the surface of the tank. This is a method often to be particularly recommended, both for convenience and security: the subject is better preserved ; the specimens are more accessible, should they be required for microscopic examination, and by immediate

ARTIFICIAL AERATION OF THE WATER.

27

transference they can always be kept in the purest available medium.

Artificial aëration of the water contained in an aquarium may sometimes be beneficially adopted, provided it can be employed without danger of disturbing delicate productions, or injuring the more tender inmates. A simple way of effecting this, and one that we employ with excellent results, is by the use of a small garden watering-can with a very finely perforated "rose," such as is used for watering choice flowers. This we fill with water taken gently from the tank, which we pour back again from a sufficient height, like a refreshing shower sparkling with vital air.

And now, gentle reader, let us hasten to the beach : the tide is near its ebb, and yonder rocks, baring their shoulders to the sunshine, seem to rest themselves in grim repose.

"All the broad bosom of the ocean keeps
An equal motion; swelling as it sleeps,
Then slowly sinking: curling to the strand,
Faint, lazy waves o'ercreep the ridgy sand,
Or tap the tarry boat with gentle blow,

And back return in silence, smooth and slow."

This is the time for work. Come, boy! the fishingbasket and the muslin landing-net-a hammer and an iron chisel. Mind, too, you don't forget the large glass jar with handles made of rope, wherein to put what specimens we find.

CHAPTER II.

"And here were coral bowers,

And grots of madrepores,

And banks of sponge, as soft and fair to eye
As e'er was mossy bed

Whereon the Wood-Nymphs lie

With languid limbs in summer's sultry hours."

SPONGES.

AMONG the most unobtrusive, but by no means the least interesting productions of the sea-shore, and, moreover, obtainable abundantly from the rocks of every beach by any one who chooses to select these elegant organisms to form a living carpet for the naked rock-work of the aquarium, are the SPONGES, a race of beings dubiously interposed in the classifiIcation of the modern naturalist between the animal and the vegetable departments of creation.

It is not known under what pressure of the ocean these delicate creatures may live*, but they are found equally in places covered perpetually by the sea, as in those which it leaves dry at every recess of the tide. They adhere to, and spread over the surface of rocks, sea-weeds, marine shells, and many other objects, to which they are so firmly attached, that they cannot

* Professor Grant, Ed. New Phil. Journ.