GENERAL NOTES ON LAMELLIBRANCHS. 373 



pods ; the pedal gland of Gasteropods is often represented by a "byssus" 

 gland, which secretes attaching threads, well seen in the edible mussel 

 (Myiihts). In oysters the foot is aljsent. 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 exhalent and inhalent 

 "siphons." 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 variations 

 of the adductor muscles afford one basis for classification. 



We may associate with the sluggish habits and sedentary life of 

 bivalves (l) 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 absence of head eyes, biting or rasping organs) with 

 the conditions of life, without being able to say very precisely what the 

 relation is. It seems to some not improbable that sluggish habits have 

 cumulative and manifold results in the course of generations ; that 

 structural changes produced by use and disuse of parts may have con- 

 stitutional consequences which affect the germ cells, that is to say, the 

 offspring. To others the adaptations seem to be most readily explained 

 as the result of the natural selection of indefinite germinal variations, 

 which arise altogether apart from function or environment. In thinking 

 about the sluggishness of most bivalves, w^e must not forget that the 

 larval trochospheres and veligers are very active, perhaps almost too 

 active, young creatures. 



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 edidis) 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 ; Dreissena polyniorpha 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 ( Unio, Anodonta, &c. ), 

 are relics of a fauna w^hich inhabited former inland seas, of which some 

 lakes are the freshened residues. 



Between the active Lima and Pecten, which swim by moving their 

 shell valves and mantle flaps, and the entirely quiescent oyster, which 

 has virtually no foot, there are many degrees of passivity, but most 

 incline towards the oyster's habit. Of course, there is much internal 

 activity, especially of ciliated cells, even in the most obviously sluggish. 

 The cockle (Cardiuin) uses its bent foot to take small jumps on the 

 sand ; the razor fish (Solen) not only bores in the sand, but may swim 

 backwards by squirting out water from within the mantle cavity ; many 



