228 LIFE BY THE SEASHORE. 



them the foot has grown up round the mouth, and is split 

 up into "arms" furnished with suckers. Except in the 

 pearly nautilus of the Pacific, there is no external shell, 

 and the structure is in many respects strangely modified. 

 Most of the cuttles live in the open sea, and they are not 

 common on the shore rocks. 



In studying the Mollusca we shall first consider the 

 Gasteropods, beginning with some old-fashioned forms, which 

 are sometimes separated from the Gasteropods, because they 

 are in many respects of simpler structure. These are the 

 species of Chiton, animals very common on our coasts, and 

 known as fossils from very early rocks. So 

 abundant are the Chitons on the shore rocks, 

 that one species at least can always be found 

 .even at the most sluggish of neap tides. They 

 live on and under stones, and are of small 

 size, being usually not more than about half 

 an inch in length, and often less. The shape 



FIG. er.-CMto ( see Fi S' 67 ) is a lon g oval > and the most 

 marginatus, marked characteristic in surface view is the 



showing's presence of no less than eight overlapping 

 eight shells, shell-plates, embedded in a tough roughened 

 mantle, which projects at the margin of the 

 plates. Remove the animals from the rocks with your 

 ringers, and you will find that they immediately begin to 

 curl up, bending the body at the junctions of the plates. 

 Watch living specimens crawl over the muddy shale, and 

 notice the slug-like movement, and the muddy track left on 

 the rock. Induce your specimens to crawl up the side of a 

 clear glass vessel, and study the under surface. In the 

 centre lies the foot, a muscular creeping surface, as in 

 limpet or snail. In front of it, and not clearly separated 

 from it, is the head, without tentacles or eyes, but with a 

 very distinct mouth-opening. By watching closely you may 

 see a brown ribbon protruded from this opening, and used 

 to scrape off the glass the small green Algse which soon 

 grow in aquaria ; thus Chiton has a radula in its mouth just 

 as the limpet has. At the sides of the foot are the gills, 

 arranged in longitudinal series, and usually about sixteen in 

 number. The posterior opening of the food canal is at the 

 extreme end of the body, as far as possible from the mouth. 



