220 THE LAMELLIBRANCHIA 



pouches ; but in all other Lamellibranchia the mouth leads directly 

 into the stomach through a short oesophagus (Fig. 201, oe), which 

 is rarely muscular (Poromya) and is sometimes nearly aborted. The 

 stomach is a large and generally laterally compressed ovoid or 

 piriform sac, more or less deeply buried in the viscero-pedal mass 

 (Figs. 207, st ; 231 and 234, st). Its walls are thin and not muscular 

 except in some carnivorous forms such as the Septibranchia. 

 The stomachal epithelium is lined with a thick but caducous cuti- 

 cular coat, visible even in the larval stage (Ostraea, Fig. 192, A) : 

 this cuticle serves to protect the secretory cells of the stomach. 



The stomach is very commonly provided with a pyloric caecum, 

 lined by a richly ciliated columnar epithelium. The caecum may be 

 long, especially in Donax, Mactra, Solen, Pholas, and Teredo, and 

 sometimes extends into the ventral part of the foot, or into the 

 mantle, penetrating into the right lobe in Anomia, the left lobe in 

 Mytilus latus. It is, however, short in some forms, e.g. in Trigonia. 

 It corresponds to the caecum of the crystalline style in certain 

 Gastropoda (Pteroceras, Fig. 75), and like it contains a cylindrical 

 product, the crystalline style (Fig. 201, cr.s), which is more or less 

 continuous with the cuticular lining of the stomach. In the 

 following forms the caecum is fused with the initial part of the 

 intestine, and communicates with it by a narrow longitudinal slit : 

 Area, Mytilus edulis, Ostraea, Pecten, the Lucinidae (Montacuta), the 

 Tellinidae and Psammobiidae, Cardium, the Unionidae, Mya, 

 Solenocurtus, and the Septibranchia. The extremity of the crystalline 

 style projects into the stomach and is gradually eroded b} r the 

 action of the digestive secretions ; the product of its solution forms 

 a sort of cement which encrusts any hard substances that may have 

 been ingested and thus protects the delicate walls of the intestine 

 from injury. Sometimes the stomach is furnished with a second 

 ventral caecum, which may be anterior, as in Mytilus, or posterior, as 

 in the Pholadidae and Teredinidae (Fig. 195). 



The liver consists of a pair of voluminous, more or less sym- 

 metrical acinous glands which occupy the whole space surrounding 

 the stomach, and may extend into the foot (Figs. 207, 222, hep). 

 Posteriorly and dorsally the liver is generally covered over by the 

 gonads. In the adult Nuculidae and Ledidae the left lobe is the 

 larger, and the coils of the intestine are situated on the right side. 

 It should be noted that, in developmental stages, the left liver lobe 

 of Lamellibranchia, like that of the Gastropoda, is larger than the 

 right (Fig. 192, l.l, r.l). The hepatic orifices leading into the 

 alimentary canal are often multiple, even in some Protobranchia, 

 but in development and in many adult forms (Solenomya, Adacnarco, 

 Modiolaria, various Erycinidae, Pseudokellya, etc.) there are only two 

 more or less symmetrical orifices. As a result of specialisation 

 these larval apertures may multiply, and various numbers are found 



