MOTION. 



441 



curious spectacle of globular bodies moving 

 in direct opposition to the force of gravity. 

 The structure of this interesting animal in a 

 mechanical point of view is worthy the pro- 

 found attention of the mathematician, as well 

 as of the anatomist and physiologist. 



SECT. IV. Annelida. The terrestrial An- 

 nelides, such as the Lumbrici or Earth-worms, 

 have the body cylindrical, and divided into up- 

 wards of one hundred and twenty segments, 

 which permit of extension and contraction, with 

 the power also of curving the trunk vertically 

 and horizontally, according to the play of the 

 muscular system. The progression of the 

 Lumbrici is aided by their retractile setae, or 

 conical spines, eight of which are attached to 

 each ring, consequently there are as many or 

 more than 8 X 120 = 960 of these setae in 

 a single worm, to assist its locomotion. As 

 many of the Lumbrici attain the length of 

 nearly twelve inches, there must be about twelve 

 rings or segments in an inch of the body 

 taken longitudinally ; now each ring being arti- 

 culated with great freedom of motion, and the 

 integuments being soft, flexible, and elastic, the 

 trunk possesses very great mobility. 



The locomotion of the Lumbrici is simple, 

 and performed in the following manner. When 

 the animal is about to advance, the head is 

 raised, and about fifteen or twenty of the an- 

 terior segments are extended and placed firmly 

 upon the plane of position ; the setae and rings 

 assist in fixing the segment in advance ; this 

 being effected, the next fifteen or twenty rings 

 are drawn forwards, and the setae are again 

 fixed in a similar manner; subsequently a third 

 and fourth series successively complete the pro- 

 gression of a step. The space taken at each ad- 

 vance depends on the energy and magnitude 

 of the animal, and the nature of the surface on 

 which the movements are performed. Some 

 Lumbrici, about six inches in length, having been 

 placed upon a smooth surface, performed a 

 distance equal to their own length at six or 

 seven complete steps, taking about one inch at 

 each advance ; this occupied nearly one mi- 

 nute of time, being a velocity of progression of 

 about thirty feet per hour. The smoothness of 

 the surface retarded the celerity of their move- 

 ments in consequence of the setae being inca- 

 pable of fixing the segments against any points 

 with sufficient power to enable them to draw 

 the succeeding segments forward ; under these 

 circumstances the mouth was observed to be 

 substituted, and to lay firm hold of the surface, 

 whilst the anterior segments were drawn for- 

 wards. The Lumbrici are capable of ascend- 

 ing a plane inclined at an angle of 45 to the 

 horizon, provided its surface presents suffi- 

 cient irregularities for the application of the 

 rings and setae. The centre of gravity of the 

 Lumbrici is very near the middle of their 

 length. The cylindrical form of the articulata, 

 and the minute dimensions of the seta?, render 

 them (with such a limited basis of support) 

 very liable to turn upon their axis, and roll 

 over on their backs, but they readily recover 

 the pendent position of the abdomen by bend- 

 ing the trunk forwards into an arch, with its 



convexity resting on the plane of motion, and 

 the heaa and anus raised above it, but inclined 

 to one side : the inclined direction of the raised 

 segments causes the animal to revolve on its 

 axis, and regain its natural position. 



When irritated, the Lumbrici contract and 

 contort the body into curves resembling in 

 form the letter S ; they appear capable of con- 

 tracting the body to one half of its entire length ; 

 in which condition the integuments present a cor- 

 rugated appearance. The Nais and Naiades are 

 swimmers. The Hremocharis walks like the 

 caterpillar of the Geometra. The Hirudines, 

 or Leeches, are more developed in a transverse 

 direction than the Lumbrici ; in these animals 

 the mouth is surrounded by a lip, and the 

 anal extremity is furnished with a flattened 

 disc, each of which is capable of causing a 

 vacuum, and the head and anus being fixed to 

 the plane of position, whilst the body is elon- 

 gated and contracted alternately, the locomo- 

 tion is effected. Leeches are well known to be 

 capable of thus ascending vertically upon the 

 smooth surface of glass, to which they adhere 

 with considerable force. 



Insecta. Apode larvae of Insects. The num- 

 ber of segments which compose the lengthened 

 cylindrical form of the Apode larvae and the dis- 

 position of the muscular system permit the trunk 

 to be moved in various directions ; to be elon- 

 gated, contracted, curved upwards, downwards, 

 or on either side, thus contributing to the pro- 

 gression of the Apodes. The cephalic, thoracic, 

 and abdominal sections, which may be consi- 

 dered as merely auxiliary organs in animals 

 furnished with arms and legs, are employed by 

 the cylindrical Apodes as the sole instruments 

 of locomotion. 



The progression of the Balaneus Nucium, the 

 Maggot of the hazel-nut, is thus performed. 

 Having first laid hold, with the mouth, of some 

 point in the plane of position, the body is con- 

 tracted and curved upon itself, and the anal 

 extremity drawn forwards ; the latter then takes 

 a fixed point for a fulcrum, and the segments 

 which had previously approximated during the 

 contraction, are again separated in succession 

 from behind forwards, causing a slight undula- 

 tion of the body in successive curves, vertical to 

 the plane of motion. The head having been pro- 

 jected forwards by the elongation of the trunk, 

 repeats the same actions, recurring as before, 

 in succession. Their progression is slow and 

 laborious, each step not being more than from 

 one-twelfth to one-fourteenth of an inch. The 

 Ciontis Scrofulariae, like the common Snail, 

 secretes a slimy substance which enables it to 

 walk on the leaves of the figvvort, on which 

 it feeds.* The larva of the Muscidae are pro- 

 vided with unguiform mandibles, with which 

 they maintain a firm hold, whilst the body is 

 contracted and dragged forwards. Other larva?, 

 as the Syrphus, use their mandibles for the 

 same purpose.f 



Pedate Larva. Many of the pedate larvae 

 of insects are furnished with six legs like the 



* DP Geer, vol. v. p. 210. 



t Kirby anil Spence, vol. ii. p. 272. 



