82 Part second. 



are brought together by means of one or two muscles, but open by an 

 elastic external ligament when the muscles are relaxed. The absence of 

 a head is a characteristic feature of this group. The protrusible "foot" 

 (Fig. 171 on the left) serves as organ of locomotion. The body is 



Fig. 171. Solecurtus strigilatus, '/o na t- size, on the left the foot, 

 on the right the respiratory tubes. 



covered on both sides by the leaf-like gills , and the latter by the two 

 mantle-flaps which secrete the two pieces of the shell. This shell is like 

 the binding of a book, the leaves of which are represented by the two 

 mantle-flaps and four gill-plates. The cilia or hairs which cover the 

 gills and the mantle, by their beating movement create a current which 

 is constantly bringing fresh water from the surroundings to the gills, so 

 that the latter are well aerated. At the same time this current serves 

 to bring microscopic animals and other food material into the mouth of the 

 animal (see p. 53). Those shells which burrow deep into the sand allow 

 a pair of long tubes (siphons) to protrude a little, and through these 

 take in and pass out a current of water (e. g. Solecurtus, Fig. 171). 

 The Bivalves are generally either fixed permanently like the oyster, or 

 they burrow to some depth into the sand; a very few can swim about 

 freely or can jump. (With the exception of Pecten the Bivalves will 

 be found in Tank Nr. 22). 



The most important example is the Oyster, Ostrea edulis (Fig. 127). 

 Everybody knows its unpretentious shell , which is usually fixed to a 

 rock by the thicker half. In their youth the Oysters swim about freely 

 in the sea, but they soon settle down and secrete a substance which 

 glues the shell to the rock. The "foot", which in most Bivalves is the 

 chief organ of locomotion and assumes considerable dimensions, becomes 

 quite rudimentary in the Oyster, where it is no longer used. Each 

 Oyster is both male and female. The eggs may number several millions 

 and ripen in summer. The young live in the mantle-cavity of the parent, 

 till their shell is sufficiently strong to allow them to "swarm" and fix 

 themselves. 



The Oyster lives in all seas with the exception of the Baltic, and 

 often makes its way up into the rivers. In Europe and North America 

 they are artificially reared on "Oyster-beds", as they are not only a 

 luxury but (especially in England and America) one of general con- 

 sumption. The number of Oysters eaten in England in a year is said 

 to be 2000 millions, while America consumes 4000 millions. Artificial 

 culture was already practised by the ancients; at the tables of Imperial 

 Rome oysters were never wanting, and epicures declared the best to 



