372 MOLLUSC A. 



quite rudimentary in Anodonta] appears ; the single adductor atrophies 

 and is replaced by two ; the foot and the gills make their appearance ; 

 the embryonic mantle lobes increase greatly, or are replaced by fresh 

 growths ; and the permanent shell begins to be made. 



After this metamorphosis, when the larva has virtually become a 

 miniature adult, no longer so liable to be swept away, it drops from its 

 temporary host to the bottom of the pond or river pool. 



CLASSIFICATION. 



Lamellibranchs are often classified as follows, the emphasis being laid 

 on the adductor muscles : 

 Order I. ISOMYA. Adductor muscles approximately equal. 



Sub-order I. Integripallia. The mantle's line of attachment to 

 the shell is not broken by a sinus into which inhalent and 

 exhalent siphons may be retracted, but in most these siphons 

 are present. Area (Noah's Ark shell), Unio and Anodonta 

 (fresh water), Lticina, Cyprina, Cardium (cockle), Cyclas 

 (fresh water), Tridacna (the largest form). 



Sub-order 2. Sinupallia. The mantle's line of attachment to 

 the shell is inflected by a sinus into which the large siphons are 

 retracted. Venus, Mya, Saxicava (a boring bivalve), Solen 

 (razor shell), Pholas (borer), Teredo (ship worm), Aspergillum 

 (watering pot shell). 



Order 2. HETEROMYA. The anterior adductor is much smaller than 

 the posterior, and siphons are rare. Mytilus (edible mussel), 

 Modiola (horse mussel), Lithodoimis (borer), Dreissena. 

 Order 3. MONOMYA. One adductor, no siphon. Ostrea (oyster), 

 Anemia, Lima, Pecten (scallop). 



Pelseneer, however, lays emphasis on the nature of the gills, and 

 classifies as follows : 

 Order i. PROTOBRANCHIA. The gill filaments, arranged in two 



divergent rows, are not reflected, e.g. , Nucula, Solenomya. 

 Order 2. FILIBRANCHIA. The gill filaments lie parallel and are directed 



ventrally ; they are reflected, and united only by ciliated 



interfilamentar bridges, e.g., Anomia, Area, Mytilus. 

 Order 3. PSEUDOLAMELLIBRANCHIA. The gill filaments are loosely 



connected by connective or vascular bridges, e.g. , Avicula, Ostrea, 



Pecten. 

 Order 4. EULAMELLIBRANCHIA. The gill filaments are bound into 



plates, as in Anodon, Cardium, Mya, Venus. 

 Order 5. SEPTIBRANCHIA. The gills form a remarkable muscular 



septum extending round the foot from the anterior adductor 



to the separation of the two siphons. Poromya and Cuspidaria. 



GENERAL NOTES ON LAMELLIBRANCHS. 



The organs which most frequently vary in other bivalves, as com- 

 pared with Anodonta, are the foot, the gills, the adductor muscles, and 

 the mantle skirt. The foot varies much in size and shape ; in Area it 

 has a flat sole-like surface which suggests the creeping foot of Gastero- 



