302 
RESPIRATION IN PEEECYPODA. 
In oiir Pelecypotla the liraiioliial iilanieuts have coiitiiiuecl to 
elongate, and to secure t'reedoin of growtli have bent back upon 
themselves, sn that the paired tilanients on each side suspended from 
their i)oint of support, which i)reviously formed an inverted V, are, by 
the retlected growth, converted into a tigure resembling the letter W. 
'file separate tilanients may, however, combine together and form a 
j)late-like, although iierforate, gill, this fusion and vascular continuity 
of the constituent tilanients being preceded by the close interlocking 
of the cilia clothing their surface 
and followed by a more or less in- 
timate interlainellar concrescence. 
The primitive tilamentous condition 
may, however, arise secondarily 
bj" the splitting nji of previousl}^ 
e.xistent lamellate gills, as is stated 
by Korschelt to be the case with 
Sph(trium. 
The posterior mantle margin, in 
the burrowing sjiecies, is often 
produced into one or a pair of 
tubular processes or siphons for 
the purpose of conveying the water to and from the gills ; the lower- 
most, known as the branchial or afferent sijihon, is encircled by a 
number of long sensitive tilanients or tentaciila; the other, known as 
the anal or efferent aperture or siphon, carries away the respired water 
after it has passed over the branchim, and is usually shorter and 
more dorsally placed, with occasionally a tilamentous termination. 
In the marine Imrrowing species, the respiratory siphons are very 
long and very extensile, Imt usually corresiiond to the de])th of the 
burrow occupied by the mollusk, although species like Anod(»ita 
rjiguea, which possess comparatively rudimentary siphons, sometimes 
live buried beneath ten inches or more of soft mud and yet preserve 
free communication with the water above by means of vertical perfora- 
tions through the soil, formed and kept open by tbe siphonal currents. 
'I'be Pelecypoda have been classihed by Pelseneer into five groups, 
based ui»on tbe amount of siiecialization the gills have undergone, 
the simplest being known as Protobranchiata and the most .sjiecialized 
as Septibranchiata, these being connected by tbe groups Filibranchiata, 
Eulamellibranchiata and P.seudolamellibranchiata ; all our British 
