220 ANNULATA. 
There are nineteen pairs of gills. They are situated 
along the sides of the thorax and are protected by the 
branchial plate of the carapace. The branchial 
chamber so formed communicates freely with 
the exterior between the legs and at the hind end, but the 
principal aperture (the cervical canal) lies at the front end 
and opens beside the mouth. 
In this there lies the scaphognathite of the 2nd maxilla 
which is said to bale or scoop the water ow¢ of the branchial 
chamber, fresh water coming in from behind and between 
the legs. Each gill consists of a central axis with lateral 
branches, covered with a very thin cuticle, ectoderm and 
mesodermic layer. In its interior the blood circulates from 
afferent to efferent branchials. 
Five of the gills are fixed to the bases of the third 
maxillipede, chela and first three legs. They are termed 
podobranchs. To the arthrodial membrane of each of 
the same appendages is attached a pair of small arthro- 
branchs ; whilst higher up, on the side-wall of the thorax, 
are found four large pleurobranchs, which are supposed 
to correspond to the four last segments.* The epipodites lie 
between each set of gills in each segment and force the in- 
coming water to pass the whole way up the gills instead of 
taking a short cut to the cervical canal. 
It is possible that the primitive arrangement was that 
of aSpodobranch, two arthrobranchs and a pleurobranch to 
each segment of the thorax, making a total of thirty-two, 
but this number persists only in numbers 5, 6 and 
7, where the thorax is broadest. The cavity has 
become narrower in front and behind, hence the last leg 
loses its podobranch and its two arthrobranchs, and the 
pleurobranchs all disappear in front of number 5: so also 
do the arthrobranchs and podobranchs in the first two seg- 
ments. With a loss of ten gills in front and three behind, 
the thirty-two is reduced to nineteen. This will be clear 
after an inspection of the diagram.t 
Respiratory. 
* It is probable that @// the gills arise on the basal joint of the thoracic appen- 
dages, but the pleurobranchs and arthrobranchs migrate during development to their 
final positions. 
+ In Astacus there is a podobranch on the second maxillipede and one arthro- 
branch above it, but only the last pleurobranch remains; thus it possesses only 18 
pairs of gills, z.e., podobranchs (6), arthrobranchs (z1) and pleurobranchs (1). 
