242 CUTTLE-FISH, MOLLUSC A, ETC. 



plants. No doubt these zoospores form a good part 

 of the food of such sedentary mollusca as oysters, 

 mussels, &c. 



The young of the oyster do not leave the folds of 

 the mother until they are capable of moving about 

 and seeking their own food. At first, therefore, oysters 

 are free-swimming, moving about by means of special 

 tufts of cilia. A single oyster will throw out myriads 

 of such embryos, which go by the name of " spat." 

 In many respects these very young oysters resemble 

 the lower Crustacea, the Entomostraca, for like them, 

 they have two shells, through which the natatory 

 gills are protruded. When the young oysters settle 

 down to the same staid life as their parents, the spat 

 is then said to " fall." In from one to three years, 

 according to circumstances, they will have reached 

 the adult stage. At the Crystal Palace the Ame- 

 rican clam (Cyprina moneta) is shown alive. Indeed, 

 there are few bivalves which cannot be healthily 

 kept in aquaria, especially if the conditions are 

 right. Two species of cockle, the edible and the 

 spiny {Cardium edule and C. eckinatuin), may usually 

 be seen in tanks. The burrowing and leaping habits 

 of the cockles are very interesting to witness. The life 

 of the young of the common species is very much like 

 that of the oyster. The recurved spines of Cardium 

 echinatum are bent in a direction contrary to that in 

 which the animal burrows, so that they do not impede 

 its habits. All the species of scallop-shells (Pecten) 



