270 LIFE BY THE SEASHORE. 



frequently used in the classification of Bivalves. Finally, 

 the gills vary much in structure, being sometimes composed 

 of free filaments, and at others woven into more or less 

 compact, plate-like structures. We shall adopt here a 

 classification based on this difference in the gills, for the 

 following reason. There can be no doubt that the existing 

 Bivalves have been derived from forms similar to, but 

 simpler than, the less specialised of existing Gasteropods. 

 The early Bivalves must have had simple, plume-like gills, 

 similar to the gills of many Gasteropods, and the more 

 complicated the gills of present Bivalves, the further have 

 they departed from this primitive condition. The most 

 natural classification seems, therefore, one based on the gills. 

 As, however, we shall consider only such Bivalves as are 

 likely to be found in the living condition, or are very 

 abundant in the dry condition on the shore, we shall 

 consider only three orders: (1) the Filibranchs, those with 

 filamentous (or thread-like) gills such as the common mussel; 

 (2) the Pseudo-lamellibranchs, those like the scallops, which 

 have gills apparently of plate-like structure, but with the 

 separate filaments so slightly attached that they fall apart 

 very readily; (3) the Eulamellibranchs, the great majority 

 of Bivalves in which the gills are firm plates, whose 

 constituent filaments cannot be readily separated from one 

 another. 



The first order includes the curious little saddle-oyster 

 and the mussels, together with other forms which need not 

 be considered here. The saddle - oyster, or silver -shell 

 (Anomia ephippium), is common under stones between tide- 

 marks, and exhibits several peculiarities of structure. The 

 shell is fragile, pearly white in colour, and often irregular 

 in shape. It consists of an upper convex valve, and a lower 

 flat one perforated by a large hole beneath the beaks. The 

 animals are extremely sedentary in their habits, growing 

 fixed to rocks, but the fixation is accomplished in a some' 

 what peculiar way. We have already explained that most 

 Bivalves possess a byssus gland in the foot, which secretes 

 a mass of silky threads serving to anchor the animal to 

 surrounding objects. The saddle-oyster also possesses this 

 characteristic gland, but both it and the foot are much 

 reduced and apparently functionless. But nevertheless the 



