GENERAL NOTES ON LAMELLIBRANCHS. 379 



Classification. — The best classification of Lamellibranchs seems to 

 be that of Pelseneer, which is based on the structure of the gills. 



Order 1. 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. Fiubranchia. — 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-lamellibraxchia. — 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. Anodonta, Venus, Pholas 

 (a boring form), Mya. 



General Notes on Lamellibranchs. 



Structure. — The organs which most frequently vary in other 

 bivalves, as compared with 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 exhalant and inhalant 

 "siphons" (Fig. 159). 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 sedentaiy 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 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. Mytilidae, 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 



