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THE SENSORI-MOTOR SYSTEM 15 



anged round a complicated system of canals through which 

 the sea- water circulates, and the larger of these canals would 

 collapse by the weight of the flesh if they were not kept open by 

 the horny fibres embedded in the latter. There is no locomotion 

 at all here, and the only mobile parts are the vibratory hairs 

 which cause the circulation of water through the canal system. 

 Such an animal as the jelly fish (or medusa) has no skeleton, but 

 it floats suspended in the sea-water, which thus supports the 

 soft, fleshy body. The latter has the shape of a bell, by the 

 expansions and contractions of which water is taken into the 

 cavity and then ejected, and so the medusa slowly moves about. 

 There is no skeleton in a common garden slug, and when loco- 

 motion occurs the front part of the broad, fleshy " foot " adheres 

 to the ground, while the latter part becomes detached and is 

 drawn forward and adheres. The front part is then detached, 

 pushed forward, and again adheres, and so on. The earthworm 

 has no skeleton, but the body is provided with muscles which 

 act so that it can be stretched out or shortened. Along the 

 sides there are little projecting bristles, which can be turned 

 forwards or backwards. When the worm burrows, the bristles 

 on the front part of the body are bent backwards and inserted 

 into the soil, and then the hinder bristles are bent backwards also, 

 and that part of the body contracts and is drawn forwards. The 

 worm then bores into the ground in front of it, swallowing the 

 soil, and stretches out the front part of its body, and gets a new 

 grip by its bristles, and so on. 



In all insects, spiders, Crustacea, etc., the skeleton is an 

 external one, consisting of a hard, horny, or limy cuticle. This 

 acts as the support for the muscles. The body and limbs are 

 jointed, or articulated, and the muscles move these parts on each 

 other in ways that are analogous to those which we are about to 

 describe in the case of the vertebrate animal. Such an exo- 

 skeleton would be very heavy if the animal were to attain a large 

 size, and it necessarily prevents growth, which can only occur 

 during the periods when the animal " moults," or casts its shell. 

 That period is one of danger for the crustacean, and so it happens 

 that these animals never have attained to a great size. When 

 the skeleton is an internal one, as it is in all vertebrates, it is 

 mechanically a more perfect structure, and so some of the species 

 in which this kind of skeleton has occurred have evolved to 

 extraordinary sizes, as in the extinct dinosaurs of the Mesozoic 



