Encyclopaedia Britannica > Volume 21, ROT-Siam

579
S E A —S E A
lation. Winds and differences of barometric pressure are,
as in inland seas, great factors in producing variable
currents. (See Baltic Sea, Black Sea, Mediterranean
Sea, Red Sea, &c.)
Partially Enclosed Seas may be (a) comparatively shallow
irregular channels through which strong tides sweep, or (b)
ocean basins cut off by barriers barely rising to the surface,
or remaining permanently submerged, in which case there
may be no break of continuity in the ocean surface to indi¬
cate the sea. Seas of the first description are related to
shallow enclosed seas, but are much affected by tides and
ocean currents; the principal are the Kara Sea of the Arctic
Ocean, Baffin Bay and North Sea of the Atlantic, Behring
Sea and Japan Sea of the Pacific. They are subject to
considerable temperature changes owing to their proximity
to land. Seas coming under the second category combine
the peculiarities of the open ocean and of deep inland seas.
The Caribbean Sea of the Atlantic, the China Sea, Java
Sea, and numerous small seas of the eastern archipelago
of the Pacific are the best examples. Their chief peculi¬
arity is that the temperature of the water instead of falling
uniformly to the bottom becomes stationary at some inter¬
mediate position corresponding to the top of the barrier.
They are usually very deep. (See North Sea, Norwegian
Sea, and Pacific Ocean.)
Other Seas.—Coral Sea, Arabian Sea, Sea of Bengal, are
names, now dropping out of use, to designate parts of the
ocean. “Sargasso Sea” is an expression devoid of geo¬
graphical meaning (see Atlantic Ocean, vol. iii. p. 20).
Firths and Estuaries.—A river entering the sea by a
short estuary flows over the surface, freshening it to a con¬
siderable extent, and, if the force of its current is not too
great, the rising tide slowly forces a wedge of sea water up
between river and river bed, withdrawing it rapidly when
ebb sets in. In a firth that is large compared with the
river falling into it, judging from results recently obtained
in the Firth of Forth,1 a state of equilibrium is arrived
at, the water increasing in salinity more and more gradu¬
ally as it proceeds seawards, the disturbing influence of the
tide becoming less and less, and the vertical distribution of
salinity more and more uniform until the river water meets
the sea, diffused through a nearly homogeneous mass with
a density little inferior to that of the ocean. Between the
extreme cases there are numerous gradations of estuary
depending on the ratio of river to sea inlet.
Deposits.—All seas within about 300 miles of continental
land, whatever may be their depth, are paved with terrige¬
nous debris, and all at a greater distance from shore are
carpeted with true pelagic deposits (see Pacific Ocean).
Marine Fauna and Flora.—The mixing of river with
sea water produces a marked difference in the fauna and
flora of seas. Where low salinity prevails diatoms abound,
probably on account of the greater amount of silica dis¬
solved in river water, and they form food for minute pelagic
animals and larvae, which are in turn preyed upon by larger
creatures. In some seas, such as the North Sea, there are
many celebrated fishing beds on the shallow banks of which
innumerable invertebrate animals live and form an inex¬
haustible food-supply for edible fishes. Naturalists have
remarked that in temperate seas enormous shoals of rela¬
tively few species are met with, while in tropical seas species
are very numerous and individuals comparatively few.
Organisms, such as the corals, which secrete carbonate of
lime appear to flourish more luxuriantly in warmer and
salter seas than in those which are colder and fresher.
The geological and dynamic aspects of seas are treated of
in Geology (vol. x. p. 284 sq.) and Geography (Physical) ;
and in Atlantic Ocean, Baltic Sea, Black Sea, Indian
Ocean, Mediterranean Sea, North Sea, Norwegian
Sea, Pacific Ocean, Polar Regions, and Red Sea the
general geographical and physical characters of oceans and
seas are described. In Meteorology some account is
given of the influence of the sea on climate, and chemical
problems connected with the ocean are discussed in Sea.
Water.
SEA-CAT. See Sea-Wolf, infra.
SEA-DEVIL. See Fishing-Frog, vol. ix. p. 269.
SEA-HORSE. Sea-horses {Hippocampina) are small
marine fishes which, together with pipe-fishes {Syn-
gnathina), form the order of Lophobranchiate fishes, as
already noticed in Ichthyology, vol. xii. p. 694. The
gills of the members of this order are not arranged in
leaf-like series as in other fishes, but form a convex mass
composed of small rounded lobes attached to the branchial
arches, as shown in the accompanying figure (fig. 1) of
the head of a sea-horse, in which the gill-cover has been
pushed aside to show the interior of the gill-cavity. Sea-
Fig. 1.—Gills of Hippocampus abdominal is.
horses differ from pipe-fishes by having a prehensile and
invariably finless tail; it is long, slender, tapering, quad¬
rangular in a transverse section, and, like the rest of the
body, encased in a dermal skeleton, which consists of horny
segments, allowing of ventral, and in a less degree of lateral,
but not of dorsal, flexion. The typical sea-horse (Hippo¬
campus) can coil up a great portion of its tail, and firmly
attach itself by it to the stems of sea-weeds or other
similar objects. The body is compressed and more or
less elevated, and the head terminates in a long tubiform
snout, at the end of which the small mouth is situated.
The whole configuration of the fore part of the body, as
well as the peculiar manner in which the head is joined to
the neck-like part of the trunk, bears a striking resem¬
blance to a horse’s head; hence the name by which these
fishes are generally known. Sea-horses are bad swimmers
and are unable to resist currents. With the aid of their
single dorsal fin, which is placed about the middle of
the fish’s body and can be put into a rapid undulatory
motion, they shift from time to time to some other object
near them, remaining stationary among vegetation or coral
where they find the requisite amount of food and sufficient
1 Mill, Proc. Roy. Soc. Ed., xiii. 29, 137, and 347.