480 



PHYLUM MOLLUSCA 



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 Zygobranchs ; the foot has a flattened 

 creeping surface : the pleural and cerebral gangha 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. Eulamei.libranchia. — 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), My a. 



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. 265). 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 Hfe of 

 bivalves— (i) the undeveloped state of the head region ; (2) the large- 

 ness 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 LameUibranchs, e.g. Mytilidaj, 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 



