LAMELLIBRANCHIATA 



579 



Protobranchiata Nucula. 



FiLiBRANCHiATA Myttlus, Pecteti. 



EuLAMELLiBRANCHiATA Ostrea, Cyclas, Cardium, Mya, Anodonta. 



Septibranchiata Poromya, Cuspidaria. 



In Fig. 395 A, B, the difference is seen between the Protobranchiata 

 with their short and simple filaments and the next two groups in which 

 each filament is greatly elongated and upturned so that descending 

 and ascending limbs can be distinguished. The contrast between the 

 Filibranchiata and the Eulamellibranchiata is expressed by Fig. 392, 

 in which a transverse section through a "gill" is shown, showing the 

 component filaments separate in the first case, save for the ciliary 

 junctions, united in the second. Lastly, in Fig. 395 C, it is seen that 

 in the Septibranchiata, the ctenidia are replaced by a horizontal mus- 

 cular partition (which moves up and down like the piston of a pump) 



Fig- 395- Vertical sections of Lamellibranchiata to show different stages of 

 development of the ctenidia. A, Protobranchiata. B, Filibranchiata and 

 Eulamellibranchiata. C, Septibranchiata. The arrows in C show the direction 

 of the flow of water through the " diaphragm ", when the latter moves down- 

 wards. After Sedgwick, from Lang. 



with apertures connecting the ventral and dorsal divisions of the 

 mantle cavity. 



The ciliation of the filaments is the same in all the first three divi- 

 sions. Even in the Protobranchiata, the ciliary apparatus for food- 

 collecting has been developed as in the rest of the group, and it has 

 been pointed out that there are ciliated discs, adjacent pairs of which 

 act as ciliary junctions and hold the filaments together to form 

 lamellae. There is, moreover, a subdivision of the mantle cavity into 

 inhalant (ventral) and exhalant (dorsal) chambers in spite of the small 

 size of the ctenidia. 



The blood system of the lamellibranchs is best explained by refer- 

 ence to that of Myttlus, the common mussel (Fig. 396). Here the 

 heart, as in Anodonta, consists of a ventricle surrounding the rectum 

 and two auricles, each of which opens into the ventricle by a narrow 

 canal and is attached by a broad base to the wall of the pericardium 



