SciEXTiFic Lectures. 223 



net Avork of muscular fibers furnisliing the broad area bj "\vhich they 

 adhere. Most bivalves and snails have a more specialized apparatus 

 called the " foot," by which they creep along. A singular feature 

 among some is the power the foot has of imbibing water ; and on 

 examination there are found to be a series of channels for this express 

 purpose. Let any one take up the common beach cockle (Natica), 

 when it is crawling over the sand, and he will see how slowly the 

 foot gradually contracts, and draws within the shell, the water which 

 had previous been absorbed slowly oozing from it. 



In another phrase we have the animals swimming through the 

 water by hydrodnamic action ; thus, in the discoid jelly fishes, the 

 water is urged from beneath the w^ater by expansions and contractions 

 of the disk. In some low forms of mollusks, e. g., salpa, the body 

 dilates, drawing in water at one opening, and then, by vigorous con- 

 traction, expelling it from another opening, and the resistance offered 

 propelling -the body. In a similar manner the common scallop, by 

 rapid opening and closing its two shells forces the body along. 



The squid, or cuttle fish, among its various modes of locomotion, 

 has the same power of ejecting jets of water, and swimming in this 

 way ; and the pointed extremity of its body is well adapted to cleave 

 the water. The paper nautilus was for a long time supposed to 

 possess the power of floating on the surface of the water, using its 

 long arms for oars, and its other two arms thrown aloft and spread, 

 as sails to catch the breeze. This beautiful story is not true, how- 

 ever, and the paper nautilus moves as all other cuttle fishes move, 

 having no power to come to the surface. Thus far, we have seen 

 movements by ciliary action, as in the infusoria, and also movements 

 of the animal wherein nearly the whole body was involved in effort. 

 Xow we are to consider animals in which the locomotive organs 

 become more specialized. In a large group of jelly fishes the body 

 is provided externally with -bands composed of many vibrating pad- 

 dles, in their movements looking very much like cilia; and these 

 cause the body to rotate, or move in a straight line. Among the star 

 fishes we find channels on the under side of the animal, which give 

 rise to a great number of little suckers, looking like so many little 

 writhing worms. Our common star fish has from 1,500 to 2,000 of 

 these suckers. They are projected like legs, and drag the body 

 slowly along. 



The cuttle fish, also, has arms furnished with suckers, but, unlike 

 the star fish, the suckers simply hold the ai-ms, so that they can find 

 points of support in their movements. 



