THE NEW YORK AQUARIUM JOURNAL. 
79 
“wood-louse” or “saw-bug,” found under logs, 
and the ‘ ‘ sand-hoppers” swarming on the ocean 
beach are familiar examples. 
Intermediate between this and the preceding 
we must put the horseshoe crab of our shores 
and the Indian ocean, which sometimes is called 
the king-crab, perhaps on account of its ancient 
lineage. It is in some respects the most highly 
organized of all the articulates, and will form a 
capital subject for your study next summer at 
the sea shore. 
Now we have ascended to the Decapods—the 
crustacean aristocracy—both beautiful and use¬ 
ful. In the long-tailed species, like the lobster 
and cray-fish (which is only a fresh water lobster), 
the abdomen—underneath which the female car¬ 
ries her mass of eggs—terminates in a broad fin, 
by the aid of which the animal swims; and so 
powerful is this tail that it is said a lobster can 
dart twenty feet backwards by one stroke. The 
forward pair of legs is prolonged into immense 
claws called chela or pincers, which they use in 
all sorts of ways. Behind these are five pairs of 
stout feet, and in front of them the complicated 
eating apparatus. Lobsters are artificially culti¬ 
vated in beds, like oysters, for food. 
You may often have been astonished, when 
just about to pick up a shell upon some sandy 
beach, to see it suddenly scamper off; and may 
remember that when you caught it you found a 
queer little fellow in it that seemed all claws. 
You had olc knd ui^ a j “soldier 
crab,’’ with his house antns back. His hinder 
parts are soft and without 'the armor of the lob¬ 
ster tribe, although he is well helmeted and 
gauntleted. So for his protection he backs into 
an empty spiral shell arid holds on, within, by 
the anchor-like end of his tail, closing up his 
front door with one of his large pincer-claws. 
When he gets too big for his castle he goes in 
search of another. If he meets a brother her¬ 
mit he offers battle, and either drags him out 
and appropriates his shell or takes a whipping 
gracefully. 
The true crabs, such as the “spider crabs” 
and “fiddlers,” have no tail at all, or none to 
speak of, and their whole structure shows that 
they were intended to walk upon the land or sea 
bottom. They carry the abdomen curled under 
the cephalo-thorax, which is here a broad shield 
covering the whole upper surface, and some¬ 
times very rough and brightly painted. Their 
eyes are bright enough, and in one group are 
carried on long eye-stalks. 
Considering their armor-like shell we might 
be puzzled to know how they have room to grow. 
It is in this way. At certain seasons the entire 
shell is cast off whole, after great wrenching. A 
new soft shell has already formed underneath, 
but the poor crab is nearly defenceless, and so 
retires to some quiet nook and waits a few days 
for his new and larger coat to toughen. 
Crustacea are abundant everywhere, but espe¬ 
cially so in tropical waters, and are invaluable 
aids in keeping the air and water pure. They 
are a part of nature’s police and scavenger force, 
for they all, big and little, live on carrion and 
the other pollutions that, if allowed to remain, 
would so taint the world as soon to make it unfit 
for habitation. Ernest Ingersoel. 
On the Taking of Food by Aquatic Animals. 
In the study of animals, it is sometimes the 
case that, while having our attention occupied 
with the orincipal function of a particular organ, 
we overlook, or consider but lightly some other- 
functions which belong to it as truly, though 
they may, when recognized, appear subordinate. 
Thus the gills of fishes are naturally, perhaps 
only associated in our minds with the process of 
respiration in these animals. The truth is, how¬ 
ever, that they serve other important uses con¬ 
nected with the taking of food (which, it must 
be remembered, is an essentially different phy¬ 
sical process in water from what it is in air); 
and the study of these, in which we are aided by 
the recent observations of a German naturalist, 
M. Jager, is highly instructive. 
When a fish snaps up an object it first opens 
its mouth and closes its gill flaps ; and opens the 
gills when it closes the mouth. When it wishes 
to reject a disagreeable morsel, on the other 
hand, it first, with closed mouth, opens the gill 
slits, and enlarges the mouth cavity, then shuts 
the gill slits and simultaneously opens the mouth. 
By narrowing the mouth-cavity throughout its 
length, it now forces out the contents ; and in 
doing so, it is driven a little backwards by the 
reaction, like a cannon when it is fired. 
If we think of it a little more closely, we shall 
see that, without the gill slits, the fish could not- 
Ssnap up any object, and so could not eat, fat 
h, be ejected. The 
would, on dosing ti*® 
reason is simply this ! On Opening, the mouth 
cavity fills with water after the manner of a pump, 
and the morsel is taken in through suction of the 
portion of water in which it floats. It can now 
be held fast in the mouth only if the water finds 
a mode of exit so narrow that the morsel cannot 
escape along with it. For this the mouth slit is 
nowise fitted, for if it be closed, so that a small 
morsel cannot escape by it, it affords no easy 
outflow for the water. But the want is fully met 
by its gill apparatus, which presents a double 
row of long, narrow slits, each of which is gen¬ 
erally a good deal longer than the mouth slit, so 
that the water can readily flow away without the 
morsel being carried off along with it. 
But again, if a fish were obliged to eject by its 
mouth the water it had taken up, it would be 
driven backwards at each bite, and have to ex¬ 
pend force wastefully in recovering its ground 
by swimming, which would be specially disad. 
vantageous in flowing water. On the contrary, 
however, as the water flows out backwards 
through the gill slits, the fish receives each time 
an impulse which drives it forwards, and the 
maintenance of its position in rapid water is thus 
rendered more easy. 
From these considerations, it becomes possible 
to explain a number of the arrangements found 
in aquatic animals, as compared with those which 
live in air. 
Still regarding the finny tribes, we find re¬ 
markably large gill slits in fishes of prey ; and 
any one who has watched a pike or a trout in 
pursuit of its prey, will have noticed how widely 
it has stretched its gill slits, so as to let the water 
flow off as freely as possible on all sides If this 
were at any moment to accumulate in the mouth 
streaming of water, 
its mouth against the current. 
cavity, tha fish’s motion would be seriously 
compromised. It may, with certainty, be said 
that all fishes with remarkably wide gill slits 
hunt their prey in long pursuit. Thus, among 
our fresh-water predaceous fishes, the pike 
makes the longest pursuit and has the widest 
gills. As a contrast we might take the gently 
feeding and nibbling plant fishes, such as barbel, 
carp, &c., which have narrow gill slits. 
A similar difference is associated with the 
As a fish always snaps with 
it receives more 
water into the mouth the more rapid the current; 
and therefore river fishes have in general larger 
gill slits than fishes which live in still water. 
Thus, too, may be explained, the remarkable 
correlation between the width of the mouth slit 
and that of the gill slits. It is clear, then, that 
the gills in fishes fulfill an important function in 
the taking of food, just as truly as lips, teeth, 
tongue, &c., in higher animals. Now it is in¬ 
teresting to inquire how those animals (am¬ 
phibia, reptiles, and certain birds and mammals) 
have been provided for that are without gill slits, 
and yet seize their food under water. A simple 
arrangement is that in which the mouth-parts for 
seizing are long and narrow, so that on the one 
hand the water has free escape to the right and 
the left, and on the other, very little water is 
compressed in the act of seizure. This explains 
the dagger and knife-shaper) ,^lls s of all swirn- 
_ _ on fish, as 
["also the ey.tr 
dolphin, 
deeply slitted snout of the crocod ile.’ 
Another substitute for the gill 
by certain arrangements in the m 
which either the morsel is sei( 
mouth slit is closed for the passage 
which permit the separation of smal 
of the mouth-water. This role is pla j 
teeth, such as those of dolphins, croc<^ 
laminated bills of geese and ducks, &c., 
baleen of whales. In these animals may als<^ 
noted a remarkable deficiency of the lips, so that 
even when the mouth is shut the teeth so to 
speak remain visible ; and there is no outer 
mouth-cavity. The lips would here only hinder 
the escape of the water. 
With regard to our gill-less amphibia, it may 
first be remarked that they seize a great part of 
their food in the air, or (which comes to the 
same thing) on the surface of the water. If they 
be observed feeding under water (which indeed 
is done almost only by newts) their awkwardness 
in comparison with fishes is very apparent. They 
cannot bring the morsel at once into their 
mouth, even though the mouth slit is compara¬ 
tively large. It may also be noticed that they 
prefer large morsels, which are seized by the 
teeth, while yet the mouth slit is widely opened; 
whereas the small pieces which a fish swallows 
with eagerness and ease are either disregarded, 
or the attempt to snap them up fails, the morsel 
being carried out again by the returning water ; 
this proves that the eating apparrtus of amphi¬ 
bians is better adapted for land than water. This 
incapability of the newts appears more clearly if 
one watch the feeding of their lame, which are 
provided with gills. In this stage of its exis¬ 
tence the newt seizes its prey with the same 
rapidity as a fish. 
t 
