. The sea-beach at ebb-tide : a guide to the study of the seaweeds and the lower animal life found between tidemarks . the older classifications, which depended 420 MARINE INVERTEBRATES upon the presence or absence of siphons, the degree of mantlefusion, the arrangement of teeth upon the hinge, the number ofadductor muscles, etc. The idea of arrangement according togill-structure is substantially this: the development of the gillfrom the simplest and most rudimentary type through successivestages to a higher, more complex, and presumably more effi-cient type, marks the natural progress or deve
. The sea-beach at ebb-tide : a guide to the study of the seaweeds and the lower animal life found between tidemarks . the older classifications, which depended 420 MARINE INVERTEBRATES upon the presence or absence of siphons, the degree of mantlefusion, the arrangement of teeth upon the hinge, the number ofadductor muscles, etc. The idea of arrangement according togill-structure is substantially this: the development of the gillfrom the simplest and most rudimentary type through successivestages to a higher, more complex, and presumably more effi-cient type, marks the natural progress or development of thepelecypod animal itself. By adopting the gill as a guide onefollows, therefore, a natural method. Upon the other hand, thepresence or absence of siphons, the shape of the foot, the numberof adductor muscles, all depend merely upon the acquired habitsof the animal, these particular features being subject to modifica-tion according to environment and changed conditions. The five orders of the Pelecypoda are: ProtobrancMata, Fili-IrancMata, PseudolamelUbranchiata, Eulamellibranchiata, Structure of the branchiae or gills of pelecypods, seen diagrammatieally in section:A, Protobranchiata; B, Filibranchiata; C, Eulamellibranchiata; D, Septibranctiiata;e,e, external row of filaments; i, i, internal row of filaments; e1, external row orplate 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 theaccompanying figure (A). Its filaments are short and not 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 theends 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 thefoot or visceral mass. The fourth type of gill is far more high
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectmarinea, bookyear1901
