Part second. 



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. 170 

 on the left) serves as organ of locomotion. The body is covered on both 



Fig. 170. Solecurtus strigilatus, Va nat. size, on the left the foot, 

 on the right the respiratory tubes. 



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. 170). 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 the scallops the Bivalves will be found in Tank Nr. 22j. 



The most importand example is the Oyster, Ostrea edulis (Fig. 127 j. 

 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 an 

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

 consumption. 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 

 come from the lake Lucrinus at Baja?. Brindisi, too, was one of the prin- 

 cipal localities , as is Taranto at the present day, for the culture of the 



