440 



MOTION. 



water, and to overcome the resistance which 

 the water opposes 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.* 



SECT. IV. 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.-f 

 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. 



Jiadidta. In the general outline of the 

 Echmodermata 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. 



t 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. 



five rays diverge at nearly five equal angles 

 from the axis of revolution. In the Sea-star 

 each ray,, according to Reaumur, is composed 

 of seven hundred calcareous plates, of which 

 there are about three thousand rive hundred 

 in the whole animal. The rays of the As- 

 terias do not possess the flexibility of those of 

 the Comatula or Gorgonia, and of themselves 

 would be insufficient to propel the animal along. 

 Nature has therefore substituted other organs 

 of progression in tubular, retractile, fleshy 

 suckers, protruded from oblique ambulacral 

 perforations, by means of which the animal 

 is dragged along the bottom of the sea or 

 upon the vertical surfaces of submarine rocks. 

 If an Asterias left to all appearance motion- 

 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 numerous 

 suckers are all soon employed in fixing and 

 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 Escalentus is one 

 of the most complicated and elaborately formed 

 species of the whole Echinodermata ; its figure 

 is spherical : the five pairs of arched ambulacral, 

 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 so 

 many inflexible levers, and the numerous 

 suckers protruding through the oblique ambu- 

 lacral foramina, as so many feet, form a double 

 set of organs for progression.* 



In the Echinus, the spines being perpen- 

 dicular to the shell, elevate its centre of gravity 

 (which, on account of its globular figure, is io 

 the centre of the shell) far above the plane 

 of motion, protect the shell from internal in- 

 jury, and increase the diameter of the whole 

 sphere, with respect to that of the shell alone, 

 by twice the mean length of the spines. From 

 the nature of their articulations, the spines are 

 capable of moving in every direction upon their 

 tubercular attachments; but these alone would 

 be insufficient to enable these animals to climb 

 the sides of submarine rocks and vertical pre- 

 cipices in search of shell-fish on which they 

 prey; but by the aid of their tubular feet, 

 which they have the power of extending be- 

 yond the spines, we behold in the Echini the 



* Grant's Outlines of Com. Anat. p. 18. 



