510 THE INVERTEBRATA 



border fonvard; 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. 



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

 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 oi 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 interlamellar septum. In one species, Pecten 

 tenuicostatus, 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. 384 D), of a very complicated structure, at regular 

 intervals all round the mantle. 



Order EULAMELLIBRANCHIATA 

 Ostrea (Fig. 375). 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 alter- 

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

 moon as in some echinoderms. Another point of physiological im- 

 portance is the great part which leucocytes play in digestion; the 

 lumen of the alimentary canal is invaded and diatoms and similar 

 bodies ingested, digested and transported by the leucocytes into the 

 connective tissue. 



A figure of the veliger larva of Ostrea is given (Fig. 351 A) to 

 show the ciliary currents by which food is obtained, the crystalline 

 style, which is revolved by the action of the cilia of the style sac, and 

 the foot, which is lost in the adult. 



