426 PHYLUM MOLLUYSCA.: 
Classification.—That of Pelseneer is based on the structure of 
the gills. 
Order 1. PROTORRANCHIA.—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. Mucula, 
Solenontya. 
Order 2. FILIBRANCHIA.—The gill filaments are greatly elongated 
and reflected, so that they consist of an ascending and a descending limb, 
e.g. Arca (Noah’s-ark shell), AZy¢zdus (edible mussel), AZodzo/a (horse- 
mussel). 
Order 3. PsEUpO-LLAMELLIBRANCHIA.—The successive gill filaments 
are loosely connected together to form gill-plates/ ¢.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, eg. Anodonta, Venus, Pholas 
(a boring form), JZya. 
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 (J/y2z/us). 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 Avxodonta. 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 AZya, 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—(1) the undeveloped state of the head region ; (2) the largeness 
of the plate-like gills, which waft food-particles 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, ¢.g. Mytilidee, 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 (AZytz/us 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 
