440 
' water, and to overcome the resistance which 
the water o to the motion of the body, 
due to its figure and velocity. In resisting me- 
dia, both the facility with which bodies im- 
mersed are supported, and the difficulty of 
moving through them, increase with the density 
of the medium: this arises from the increased 
resistance which the particles oppose to their 
displacement; hence, according to Borelli, 
fishes expend, in order to acquire a given velo- 
city, nearly twice as much animal power as 
birds, but are supported in the fluid without 
any exertion, whereas birds, as we have seen, 
are obliged to use considerable force to sustain 
themselves in the air.* 
Secr.1V. Progression on solids—The pro- 
gressive motions of animals on solids are ac- 
complished with much less expenditure of mus- 
cular action than is employed by animals in 
swimming or flying. In the various movements 
of animals upon solids the reaction of the 
ground is always equal and opposite to the 
quantity of muscular action impressed on it.+ 
The velocity being equal, muscular action in- 
creases as the resistance of the solids decreases; 
hence the augmentation of labour in walking 
on a loamy or soft soil, such as the sands on 
the sea-shore. If an animal were projected 
into space, and moving in an unresisting me- 
dium, no effort of its limbs would ever enable 
it to change either the velocity or direction of 
the motion of its centre of gravity. From 
these dynamic considerations we perceive the 
importance of a surrounding resisting medium 
to animal progression. We shall now trace 
the modes of terrestrial progression from the 
lower forms of the animal kingdom succes- 
sively through the intermediate groups, to man. 
Radiata.—In the general outline of the 
Echinodermata we observe great diversity in 
the structure of the organs of support and 
locomotion. The Crinoidea belong most gene- 
rally to those forms of the Echinodermata 
which are permanently fixed. Amongst the 
Asterioidea, in the Comatula, the locomotive 
organs consist of long, complicated, flexible arms 
radiating from a common centre, and subdivid- 
ing into numerous filaments covered with spines, 
which perform the office of so many legs, 
enabling the animal to drag itself along 
_the bottom of the sea, or of so many tenta- 
cula to lay hold of surrounding solids or to 
seize their prey. In the Ophiura the rays are 
of considerable length, being composed of a 
great number of pieces curiously imbricated 
and connected together by ligaments; they are 
flexible and moveable in every direction, and 
act not only as legs for crawling on the ground 
but also as fins, which, by a kind of undula- 
tory movement, enable the animal to swim 
during short intervals. In the Asterias the 
* See Borelli, p. 260. 
+ Barthez, in opposition to Euler, Borelli, and 
others, denies that reaction is the cause of progres- 
sive motion on solids, but in the explanation which 
he gives of it it is difficult to understand his reason- 
ing, without taking into account the resistance of 
the ground, 
MOTION. 
five rays diverge at nearly five equal ar 
from the axis of revolution. In the Se 
each ray, according to Reaumur, is compe 
of seven hundred calcareous plates, of which 
there are about three thousand five hundred 
in the whole animal. The rays of the . 
terias do not possess the flexibility of the 
the Comatula or Gorgonia, and of themselve 
would be insufficient to propel the animal ale 
Nature has therefore substituted other org 
of progression in tubular, retractile, fi 
suckers, protruded from oblique ambala 
rations, by means of which the animal — 
Is dragged along the bottom of the sea or 
n the vertical surfaces of submarine rocks. 
If an Asterias left to all appearance mot 
less and inanimate by the retiring waves, — 
be picked up from the beach, and placed in” 
a large glass jar filled with sea-water, an asto- 
nishing spectacle will be observed. “ Slowly,” 
says Professor Rymer Jones, “ the rays ex- 
pand to their full stretch; hundreds of feet 
protrude through the ambulacral apertures, and 
each apparently possessed of independent ac- 
tion, fixes itself to the sides of the vessel; as 
the animal begins its march, the nu is 
suckers are all soon employed in fixing 
detaching themselves alternately, some remain- 
ing adherent, whilst others change their posi- 
tions; and thus by an equable gliding motion 
the star-fish climbs the side of the glass.” ‘The 
progression of the Asterias is laboured and 
exceedingly slow, and ill adapted for traversing 
such surfaces as the rough shingles of the sea~ 
shore. ‘ 
Echinida.—The Echinus Esculentus is one 
of the most complicated and elaborately formed 
species of the whole Echinodermata; its figare 
is spherical : the five pairs of arched ambulaeral, 
and five pairs of tubercular columns, are joined 
to each other by zigzag sutures. The nume- 
rous spines are connected to the tubercles by a 
ball and socket articulation. According to Dr, 
Grant, the skeleton is composed of more than 
ten thousand pieces, the spines acting as 30 
many inflexible levers, and the numero 
suckers protruding through the oblique ambu 
lacral foramina, as so many feet, form a ¢ 
set of organs for progression.» 3 
In the Echinus, the spines being perpen: 
dicular to the shell, elevate its centre of gravit 
(which, on account of its globular figure, is i 
the centre of the shell) far above the plam 
of motion, protect the shell from inte val i t 
jury, and increase the diameter of the whe 
sphere, with respect to that of the shell alor 
by twice the mean length of the spines. Fro 
the nature of their eepanne ss the spines ; 
capable of moving in every direction upon th 
saheroutce ceuchies but these alone ¥ o 
be insufficient to enable these animals to eli 
the sides of submarine rocks and vertical p 
cipices in search of shell-fish on which @ 
prey; but by the aid of their tubular feet, 
which they have the power of exten 
yond the spines, we behold in the 
> eh ini 
* Grant’s Outlines of Com. Anat, p. 18. _ 
a 
ity 
