420 



MARINE INVERTEBRATES 



upon the presence or absence of siphons, the degree of mantle 

 fusion, the arrangement of teeth upon the hinge, the number of 

 adductor muscles, etc. The idea of arrangement according to 

 gill-structure is substantially this : the development of the gill 

 from the simplest and most rudimentary type through successive 

 stages to a higher, more complex, and presumably more effi- 

 cient type, marks the natural progress or development of the 

 pelecypod animal itself. By adopting the gill as a guide one 

 follows, therefore, a natural method. Upon the other hand, the 

 presence or absence of siphons, the shape of the foot, the number 

 of adductor muscles, all depend merely upon the acquired habit ; 

 of the animal, these particular features being subject to modifies 

 tion according to environment and changed conditions. 



The five orders of the Pelecypoda are: Protobmnchiata, Fill 

 brancJiiata, Pseudolamellibranchiata, Eulamellibranchiata, Septi 

 branchiata. 



Structure of the branchiae or gills of pelecypods, seen diagrammatically in section : 

 A, Protobranchiata ; B, ffilibranchiata ; C, Eulamellibranchiata ; D, Septibranchiata ; 

 e, e, external row of filaments ; i, i, internal row of filaments ; ?, external row or 

 plate folded back ; i', internal row folded back ; /, foot ; m, mantle ; s, septum ; 

 v, visceral mass. 



The first includes the simpler type of gill as represented in the 

 accompanying figure (A). Its filaments are short and not reflected. 

 The second, represented in Fig. B, has the filaments long, reflected, 

 and connected (each filament to its adjacent ones) by means of sur- 

 face cilia. The third type of gill resembles the last except that the 

 ends of the filaments of the outer gill are attached to the mantle, 

 and the ends of the filaments of the inner gill are attached to the 

 foot or visceral mass. The fourth type of gill is far more highly 



