504 



THE INVERTEBRATA 



have a surprising power of taking up solid particles. In the oyster, 

 it may be mentioned, there is an extraordinary abundance of 

 leucocytes which wander here, there and everywhere, through the 

 body. It is claimed that they enter the stomach and digest diatoms 

 and other food particles there, wandering over the body afterwards, so 

 that they play a unique part in digestion and transport of food. 



The lamellibranchs are most conveniently classified by the structure 

 of their ctenidia. We have firstly three groups which can be arranged 

 in an evolutionary series, showing the ctenidia to become larger, 

 more complex and solid organs. Lastly there is an isolated group, the 

 Septibranchiata, in which the habit of life has completely changed 

 and the ctenidia have practically disappeared : 



Protobranchiata Nucula. 



FiLiBRANCHiATA MytUus, Pecteu. 



EuLAMELLiBRANCHiATA Ostrea^ Cyclos, Cardium, Mya, Anodonta. 



Septibranchiata Poromyay Cuspidaria. 



In Fig. 370 A, B, the diff'erence is seen between the Protobranchiata 



A ^ v> B L/ V) c 



Fig. 370. 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 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. 367, 

 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. 370 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) 

 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- 



