LAMELLIBRANCHIATA 585 



the ordinary lamellibranch method but by swimming. The two valves 

 are unequal, the right being larger and more convex, and the animal 

 rests on this valve; in P. opercularis the valves are almost equal. In 

 swimming the valves open and close very rapidly, forcing out the 

 water between them. Usually the water is forced out dorsally on each 

 side of the hinge line and the animal moves with the free ventral 

 border forward ; but on sudden stimulation the current passes out 

 directly ventrally and the hinge line becomes anterior. There is a 

 single large adductor muscle: this is divided into two parts and the 

 larger of these serves for the rapid contractions which cause swimming 

 movements ; the fibres are transversely striated ; the smaller part has 

 fibres which are capable only of strong long-continued contraction 

 and keep the valves closed. (Cp. Chapter iv, p. 143.) 



The foot is very much reduced, but it has nevertheless a distinct 

 function, that of freeing the palps and gills from sharp and disagree- 

 able foreign material; in the larva it is used actively in locomotion. 

 The ctenidia, while resembling the typical filibranch gill of Mytilus 

 in general, differ in the possession of two kinds of filaments and in the 

 vertical folding of the gills. The larger principal filaments lie at the 

 bottom of the troughs between successive folds and the descending 

 and ascending limbs of each principal filament are connected by a 

 sheet of tissue, the inter latnellar septum. In one species, Pecten 

 tenuicostatiis, there are organic connections between filaments instead 

 of ciliary junctions only, and the existence of this condition is a valid 

 criticism of the classification of the lamellibranchs by ctenidial 

 structure. 



Pecten is hermaphrodite. The ovary has a very vivid pink colour 

 when the eggs are ripe. The testis lies behind it and is cream-coloured. 

 The remaining feature to be noted is the presence of a large series of 

 stalked eyes (Fig. 409 D), of a very complicated structure, at regular 

 intervals all round the mantle. 



Order EULAMELLIBRANCHI ATA 



Anodonta (Figs. 392 C, 397). Many of the characters of this fresh- 

 water genus are described above. 



Ostrea (Fig. 400). In this form the adult is always fixed by the left 

 (the larger) valve. As in Pecten, there is only one adductor muscle 

 (the posterior) in the adult (but the spat possesses two equal muscles), 

 and this is divided into two parts, one with striated the other with 

 non-striated fibres. The foot has disappeared entirely ; the two auricles 

 are fused together. Of great interest are the reproductive habits : it 

 has been established that individuals of O. edulis function alternately 

 as males and females. Spawning tends to take place at full moon as 



