THE SUBSTRATUM 



163 



firm mixed sand and mud that is not ex- 

 posed to the direct pounding of the waves. 

 Animal communities on rock surfaces con- 

 tain a large number of sessile forms such 

 as sponges, hydroids, and anthozoans, bryo- 

 zoans, mollusks, and urochordates. Other 

 animals in great variety crawl over these 

 and take refuge in the interstices between 

 their bodies. Many sessile animals also Uve 

 on the firm physical extension of the bottom 

 furnished by attached or rooted vegetation, 

 and other animals move actively over such 

 plants. 



The burrowing habit is more common in 

 muddy sand than in either pure shifting 

 sand or in solid rock, although speciaUzed 

 burrowers occur in both these substrata. 

 The sand burrowers (p. 161) must be able 

 to dig rapidly to keep covered in a sub- 

 strate that shifts quickly. Some of the dig- 

 gers in muddy sand— the lugwonn {Areni- 

 cola), for example— also dig rapidly. Solid 

 rock shelters a mixed lot of boring forms: 

 boring sponges, annehd worms, lamelli- 

 branchs, sea urchins, barnacles, and iso- 

 pods. They are usually found in the softer 

 rocks, but the boring sea urchins can pene- 

 trate even lava and metamorphic rock. 

 Representatives of some of these also attack 

 wood, notably Teredo, the moUuskan "ship- 

 worm" of the family Pholadidae. Many ani- 

 mals that lack the power to bore for them- 

 selves occupy burrows of others, and many 

 more live in the natural furrows furnished 

 by the crevices between stones. 



The rock-boring habit appears to be ab- 

 sent in fresh-water communities so far as 

 the firmer rocks are concerned. Certainly, 

 it is even less well developed than in the 

 sea. A wide variety of unrelated forms bur- 

 row into softer substrate and use a number 

 of burrowing mechanisms. Perhaps the 

 densest animal community of the fresh wa- 

 ter is that formed on and among stones in 

 moderately swift streams where relatively 

 little burrowing occurs. A dense population 

 of tubificid worms may occur in the surface 

 layer of mud with high organic content. 

 These small annelids are shallow burrowers 

 that live in tubes from which they protrude 

 the posterior end and wave it actively back 

 and forth, probably as an aid in respiration. 



Water, even shallow water, serves as a 

 protective substratum against the invasion 

 of many land predators. Dwellers in vege- 

 tative islands of cattails (Typha) and other 

 swamp plants are protected by the sur- 



rounding moat of water, the more so the 

 more permanent the water moat. 



LAND SURFACE 



The physical character of land surface is 

 an important factor in the ecology of land 

 animals. Mammals dwelling on rocky 

 ground tend to have resistant, nonskidding 

 feet that help to make them sure-footed 

 even on difficult terrain. This is the more 

 important, since rocky habitats are fre- 

 quently associated with the steeper moun- 

 tain slopes. The feet of nonburrowing 

 animals that five mainly on soil approach 

 the generalized condition characteristic of 

 their group. Specializations occur, among 

 them the tendency of ratite birds (emu, 

 rhea, and ostrich) for a reduction in the 

 number of toes to three or even to two, and 

 among cursorial mammals towards smaller 

 feet, Hkewise with fewer toes. Heavy ani 

 mals, living on a soft substrate, tend to de- 

 velop larger feet than those of closely 

 related forms from firmer ground. The 

 webbed feet of birds function to keep their 

 possessor from sinking in mud as well as to 

 swim. The extended toes of the jacana en- 

 able these birds to walk on floating leaves 

 of water plants. 



Mammals that five mainly on a habitat 

 with a soft substratum have noticeably 

 large feet, whether they live in marshes 

 (moose) or run on snow, as does the snow- 

 shoe rabbit, or on loose sand (Gazella lo- 

 deri) . Many different kinds of animals have 

 become adapted to move over loose sand. 

 Tenebrionid beetles of extensive sands are 

 supported by widened tarsi extended by 

 chitinous hairs or have their mesothoracic 

 and metathoracic legs greatly lengthened 

 and so run over loose sand somewhat as 

 water striders do on the surface film of 

 water (Faussek, 1907; Gebien, 1920). 

 Among their sand-dwelling species, four dif- 

 ferent famiUes of hzards show convergent 

 development of lateral rows of scales or 

 fringes on their toes. Different genera of 

 snakes, living in different parts of the world, 

 have independently become "side winders" 

 as an adaptation to locomotion over loose 

 sand (Fig. 35). The jumping mouse 

 (Dipus) has lateral hairs from the soles of 

 its feet and the sand grouse (Sijrrhaptes) 

 has feathered toes and a web. The toes of 

 some species of grouse are extended during 

 winter months by a curious fringe of homy 

 points that act as snowshoes. These are 



