34 



THE VOYAGE OF H.M.S. CHALLENGER. 



The facts and instances above referred to may be summed up in the following- 

 table : — 



II. PELECYPODA. 



The arrangement of the gills in Malletia, in the " Cryptodon " of the Challenger, 

 and in the entire group of " Anatinacea" formed hy Poromya, Silenia, and Cusjndaria, 

 prompted me to study their structure in a large number of other Pelecypoda. I shall 

 here summarise the results of my comparative investigation. 



It has been already indicated that among all the known gills of Pelecypoda, those 

 of Malletia have the simplest structure, and most closely approach the gills of 

 Cephalopods and Gastropods (such as Fissurella and Haliotis). They may be regarded, 

 then, as the type m.ost closely resembling the primitive gill of Pelecypoda. 



It is possible, in fact, to derive from this type all the other forms of gill found in 

 the group. One can readily understand that with further specialisation, that is to say, 

 with increase of surface for blood exposure, these organs must needs have their gill- 

 lamellae elongated into filaments. But the two rows of lamellae being then compressed 

 l>etween the mantle and the visceral mass, they must, in order to elongate and become 

 transformed into filaments, extend towards the ventral side, parallel to one another. 

 This is what occurs in the animals allied to Malletia (PI. IV. fig. 10 BB), in the 

 entire group of Arcidse. 



