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, ete. 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 habits 
of the animal, these particular features being subject to modifica- 
tion according to environment and changed conditions. 
The five orders of the Pelecypoda are: Protobranchiata, Fili- 
branchiata, Pseudolamellibranchiata, Eulamellibranchiata, Septi- 
branchiata. 
Structure of the branchie or gills of : pelecy pods, seen diagrammatically i in section: 
A, Protobranchiata; B, Filibranchi Cc, hiata; D, Septibranchiata ; 
e,e, external row of filaments; i, i, incernal row of filaments; ¢, external row or 
plate folded back; 7’, internal row folded back; f, 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 
