Metazoa		Bivalvia
MOLLUSCA		BivalvE Gill Morphology

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Bivalve Gill Morphology

MOLLUSCA 
BASAL MOLLUSCA
|--Diasoma
|  |--Rostroconchia  
|  `--BIVALVIA  
|     |--Protobranchia  
|     `--HETERODONTA  
`--CYRTOSOMA
   |--CEPHALOPODA
   `--GASTROPODA 
      |--Eogastropoda  
      `--APOGASTROPODA
         |--Caenogastropoda
         `--HETEROBRANCHIA
            |--Opisthobranchia
            `--PULMONATA

Bivalve gills are (usually) flat, thin sheets of tissue, attached to the two sides of the visceral mass and/or to the proximal part of the foot.  Pojeta (1987).  They hang down into the mantle cavity on either side of the foot (see illustration). They are used both for respiration and for food gathering.

Gills have traditionally been used as an aid to classification of Bivalves. There are four basic gill types, which are represented in the following diagram as transverse cross-sections. The following is based on a compilation from several web pages (Bivalvia, Bivalves) and especially Moore et al. (1952: 410).  

Protobranch type. These are small and leaf like. They have an unmodified appearance like that of gastropods, chitons, and cephalopods, and, one may assume, Hecionelloids. These are occur only the most primitive bivalve groups, the Palaeotaxodonta and Cryptodonta, which are united by recent workers and distinguished from all later forms.

Filibranch type. These form lamellar sheets of individual filaments in a "W" shape. They hang downwards into the mantle cavity but have their terminal portions bent upwards. These characterize many of the Pteriomorpha

Eulamellibranch type These have the same "W" shape as filibranch type, but with cross partitions laterally joining the filaments to create water filled cavities between them. This is the most advanced gill type, and also the most common, found in by far the largest number of species of bivalves.

Septibranch type. These gills are only found in the Poromyacea, a superfamily of rock borers (Order Pholadomyoida), and are clearly an adaptation to the unusual lifestyle of their owners. They run transversely across the mantle cavity forming a sort of partition that divides the mantle cavity into upper and lower sections, and almost enclosing the internal chamber, but maintains only a small connection with the outer cavity.