THE WONDERS OF LIFE 



subdermal muscle divides into two layers — an outer 

 deposit of concentric muscles and an inner layer of 

 longitudinal muscles; in the cylindrical worms (nema- 

 todes, sagittae, etc.) the latter fall into four longitudinal 

 bands, one pair of upper (dorsal) and a pair of lower 

 (ventral) muscular bands. At those parts of the body 

 which are especially used for locomotion the muscle is 

 more strongly developed, as in the belly-side of the 

 crawling worms arid moUusks. This muscular surface 

 develops into a kind of fleshy "foot" {podium); it 

 assumes a great variety of forms in the various classes 

 of moUusks. In most of the snails which creep on the 

 solid ground it grows into a muscular "fiat -foot" 

 (gasteropoda); in the mussels which cut like a plough 

 through the soft slime it forms a sharp "hatchet-foot" 

 (pelecypoda). The keel -snails (heteropoda) swim by 

 means of a "keel-foot," which works like the screw 

 of a ship ; the floating - snails (pteropoda) swim un- 

 steadily (like butterflies flying) by means of a pair of 

 head-folds, which develop from the side of the anterior 

 foot-section. In the highest moUusks, the cuttle-fishes 

 (cephalopoda), this fore-foot divides into four or five 

 pairs of folds, which grow into long and very muscular 

 "head-arms"; the numbers of strong suckers on the 

 latter have also special muscles. In all these non- 

 articulate moUusks and vermalia hard skeletons are 

 either altogether wanting or (like the external sheUs of 

 the moUusks) they have no functional relation to the 

 motor muscles. It is otherwise in the higher animals, in 

 which we find this relation to a solid jointed skeleton 

 that becomes a passive motor apparatus. 



The higher groups of the animal kingdom in which a 

 characteristic solid skeleton is developed and forms an 

 important starting-point for the muscles, as well as a 

 support and protection for the whole body, are the three 

 stems of the echinoderms, articulates, and vertebrates. 



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