GENERAL NOTES ON LAMELLIBRANCHS. 407 



Order 3. PSEUDO-LAMELLIBRANCHIA. The successive gill filaments 

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

 Ostrea (oyster). 



Order 4. EULAMELI.IBRANCHIA. The separate filaments are no 

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

 majority of Bivalves are included here, e.g. Anodonla, I'enus, Pholas 

 (a boring form), J\Jya. 



GENERAL NOTES ON LAMELLIBRANCHS. 



Structure. - -The organs which most frequently vary in other 

 bivalves, as compared \vith Anodonta, are the foot, the gills, the 

 adductor muscles, and the mantle skirt. The foot varies much 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 an interesting 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, however, 

 much 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 cxhalant and inhalant 

 "siphons" (Fig. 195). These siphons sometimes attain a considerable 

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

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

 variations of 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-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 head-eyes, biting or rasping organs) 

 with the conditions of life. In other words, these characteristics may be 

 regarded as adaptations resulting from the action of natural selection on 

 germinal variations. In thinking about the sluggishness of most bivalves, 

 we must not forget, however, that the larval trochospheres and veligers 

 are very active, perhaps almost too active, young creatures. 



In some Lamellibranchs, e.g. Mytilidee, small eyes occur on the head ; 

 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 

 slowly up from estuary to river, from river to lake ; Dreissenia poly- 

 morpha has been carried on the bottom of ships from the Black Sea to 

 the rivers and canals of Northern Europe ; and it is likely that aquatic 

 birds have assisted in distributing little bivalves like Cyclas ; (b) on the 

 other hand, it is more probable that the fresh-water mussels (Uin'o, 



