426 PHYL UM MOLL USCA. 



Classification. That of Pelseneer is based on the structure of 

 the gills. 



Order i. PROTOBRANCHIA. There are two simple posterior gills, 

 quite similar to those of Zeugobranchs ; the foot has a flattened creeping 

 surface ; the pleural and cerebral ganglia are distinct, e.g. Nucula^ 

 Solenomya. 



Order 2. FILIBRANCHIA. The gill filaments are greatly elongated 

 and reflected, so that they consist of an ascending and a descending limb, 

 e.g. Area (Noah's-ark shell), Mytilus (edible mussel), Modiola (horse- 

 mussel). 



Order 3. PSEUDO-LAMELLIBRANCHIA. The successive gill filaments 

 are loosely connected together to form gill-plates, e.g. Pecten (scallop), 

 Ostrea (oyster). 



Order 4. EULAMELLIBRANCHIA. The separate filaments are no 

 longer discernible ; the gills form double flattened plates. The great 

 majority of Bivalves are included here, e.g. Anodonta, Venus^ Pholas 

 (a boring form), Mya. 



GENERAL NOTES ON LAMELLIBRANCHS 



Structure. The organs which show most variety in bivalves are 

 the foot, the gills, the adductor muscles, and the mantle skirt. The 

 foot shows much diversity in size and shape ; the pedal gland of 

 Gasteropods is often represented by a "byssus" gland, which secretes 

 attaching threads, well seen in the edible mussel (Mytilus). The gills 

 show a series of gradations, from a slight interlocking of separate gill 

 filaments to the formation, by complicated processes of " concrescence," 

 of plate-like structures such as those of Anodonta. These processes 

 are more closely related to the method of nutrition than of respiration, 

 which, indeed, is probably largely performed by the mantle skirt. The 

 mantle skirt is often united to a greater or less extent inferiorly, and is 

 often prolonged and specialised posteriorly to form exhalant and inhalant 

 " siphons" (Fig. 223). These siphons sometimes attain a considerable 

 length ; they occur especially in forms such as Mya, which live buried 

 in sand or mud, or which burrow in wood or stone, e.g. Pholas. The 

 diversities in the adductor muscles afford one basis for classification. 



We may associate with the sluggish habits and sedentary life of 

 bivalves (i) the undeveloped state of the head region ; (2) the largeness 

 of the plate-like gills, which waft food -par tides to the mouth ; and (3) 

 the thick limy shells. We may reasonably associate these and other 

 facts of structure (e.g. the rarity of anterior eyes, biting or rasping 

 organs) with the conditions of life. 



In some Lamellibranchs, e.g. Mytilidae, small eyes occur at the base 

 of the most anterior filament of the inner gill-plate ; in some other 

 cases they are present in the larva, but not in the adult. 



Habit. Most bivalves, as every one knows, live in the sea, and 

 their range extends from the sand of the shore to great depths. They 

 occur in all parts of the world, though only a few forms, like the edible 

 mussel (Mytilus edulis]^ can be called cosmopolitan. Some, such as 

 oysters, can be accustomed to brackish water. The fresh-water forms 

 may have found that habitat in two ways (a) a few may have crept 



