LOCOMOTION IN FLUIDS. SWIMMING. 183 



situated, as in the Vertebrata, amongst the muscles, and covered by them. 

 These internal muscles of the Annulosa are really a highly developed system 

 of subcutaneous muscles, connected with the calcareous, horny or chitinous, 

 coriaceous, or soft, integument, as the case may be ; they are homologous with 

 the panniculus carnosus or hypodermal muscles of the'Vertebrata, and have 

 no relation to the skeletal muscles of the last-named animals. The order in 

 which the limbs of the higher Annulosa move is absolutely definite for each 

 class of these animals, and differs, according to the number of the limbs ; thus, 

 the Insects, having six legs, move after one mode ; the Spiders, having eight 

 legs, follow another; the Crustaceans, some of which have ten legs, have 

 another ; and the Myriapods, or many-footed creatures, a fourth mode. With- 

 out diagrams these could hardly be made intelligible. In the Annelids, or 

 Worms, the movement over solid bodies is accomplished, either by fixing the 

 anterior extremity with the mouth, and drawing up the hinder one, and so 

 on continuously, as in the leech ; or a holding power is obtained by minute 

 setae, or bristles, set outwards and backwards, as in certain worms. In 

 these cases the extension of the body in a longitudinal direction is accom- 

 plished by the contraction of numerous circular muscular fibres, which sur- 

 round the body ; whilst its required contraction in length is brought about by 

 the relaxation of these circular fibres, and the shortening of other longitudinal 

 bands. Caterpillars move on a similar principle, but are provided with broad 

 suctorial or clasping posterior feet, as well as with the characteristic three 

 smaller, pointed and prehensile, anterior pairs. The numerous special con- 

 trivances exhibited in the extremities of the feet of the perfect insects and 

 spiders, would require volumes to describe ; sometimes they present booklets 

 (beetles), sometimes suctorial apparatuses (flies), and sometimes special adap- 

 tations, as in the spider, for holding upon webs. A great variety of special 

 modes of locomotion may also be here alluded to, as the jumping of certain 

 spiders the astonishing leap of the flea, and the peculiar sideward mode of 

 progression of certain spiders, and of the crabs ; but for illustrations of these, 

 and similar cases, reference must be made to works on Natural History. 



Amongst the Annuloida, the Scolecida present a number of creeping animals. 

 In the Echinodermata, which are entirely marine, there is in many species, as 

 in the Sea-eggs or Echini, a remarkable power of locomotion over the bottom 

 of the sea, accomplished sometimes by so-called spinigrade progression, that 

 is, by the motion of long spines, articulated with their shell, and moved by 

 little muscles ; in other cases, as seen in Star-fishes, a cirrhigrade progression 

 is performed by the protrusion and working of numerous suctorial tubular 

 cirrhi or feet, which are projected by being filled with fluid, forced into them 

 by special muscular sacs lodged in the interior of the animal ; by means of 

 these, the star-fish will even creep up the glass side of an aquarium. In the 

 soft Echinodermata, the Holothurida, the progression is of a mixed character, 

 being cirrhi-vermigrade. 



The Coelenterata have no power of locomotion over solids, being entirely 

 swimming animals ; but amongst the Protozoa, the suctorial mode of progres- 

 sion over solids is exemplified in the Rhizopods, as in the simple proteiform 

 Ainoaba. 



Locomotion of Man in Fluids. 



Owing to the lighter specific gravity of the human body, when the 

 lungs are expanded, as compared with water, man is able to swim in 

 that element, whether salt or fresh, with a small part of his frame 

 above the surface. When the lungs are fully inflated, the body is 

 lighter than water; after a complete expiration, it is heavier; but in 

 an ordinary expiration it is about the same weight, bulk for bulk: 

 hence, when the chest is fully inflated, a man may float with a small 

 part of his body above water; but by a slight muscular effort, the 

 head may be so thrown back, that, in this state, the mouth, nose, and 

 eyes remain above the surface, but any unusual expiratory act is 



