PONTON : NATURAL ENQINEERINO. 2? 7 



Under tlio liinge in eacli valve is a small spoon shaped process. These 

 processes penetrate deeply into the wing shaped muscles of the mantle and 

 foot. When the animal commences an excavation, the elastic muscular baso 

 of the foot is pressed against the block of wood or stone to be perforated, 

 and having thus gained a point of support the animal begins to turn. 

 Slowly the minute particles are rubbed, or rather, so to say, lichecl off until 

 in process of time the burrow reaches a depth sufficient for the protection of 

 the a-nimal. In cases where the shell possesses sharp teeth as in the Pholades 

 for instance, when the animal has excavated to a certain depth, by means of 

 its foot, the valves come into play and materially assist in the enlargement of 

 the burrow. 



The great expansibility which it is necessary the foot of these animals 

 should possess to enable them to perform their boring operations successfully^ 

 is owing to the presence in its interior of a curious organ called the Hyaline 

 stylite. The existence of this extraordinary body has been long known, but 

 its true nature was not discovered until the year, 1858, when the much vexed 

 question as to its use in the economy of the animal was at length set at rest by 

 the investigations of Mr. Eobertson, of Brighton, who discovered that its sole 

 object was to add elasticity to the foot. The stylite is cylindrical, about the 

 twentieth of an inch in diameter at its thickest part, and generally about an 

 inch in length, it is quite transparent and highly elastic j it lies along the 

 sole of the foot, in a case or sheath of muscles. The base of this glass-like 

 spring is contained in a cup held in by the border muscle of the foot, and 

 the socket containing the point is a horny cup, controlled by the muscles of 

 the sheath which are connected with those of the back. The narrow end of 

 the spring is spiral, and it is enclasped by the flaps of the socket. 



Still it was not enough that these animals could scoop out their holes, 

 they must have some means of getting rid of the particles removed. Tliis 

 is thus performed ; the particles of wood or stone are collected into the 

 mantle, and the foot then suddenly swells, and forces the particles mixed 

 with water through the siphons and they are thus expelled. Such is the 

 mode employed by these engineers of nature to form their excavations ; and 

 thus the cycle is perpetually kept up. The coral |)olypes, nature's builders, 

 on the one hand, forming reefs, islands, and continents; the moUusca 

 nature's engineers, on the other hand, destroying piers and breakwaters, 

 undermining rocks and cliffs. But whether for creation or destruction, still 

 ever wonderful and ever perfect are the ways in. which nature works. 



