126 Report of the Microscopical Section, [Sess, 
five, called pleurobranchiz, while attached to the side of the 
thorax, above the basal joint of the last ambulatory limb, is a 
single gill, called also a pleurobranchia, thus making the — 
ere eighteen. They may be summarised thus: 3 ambu- 
latory limbs, the great chela, and the third maxillipede, each é 
with 3 branchiz, making together 15; the second maxillipede 
with 2, and the pleurobranchia apart 1=18. In addition, | 
there are one or two rudimentary branchiz. 
Each of the podobranchiz consists of two parts: the plume, — 
a feathery-like appendage, and the lamina, a broad thin plate — 
plaited into about a dozen folds; while the arthrobranchie 
and the pleurobranchize have only the plume or feathery 
part. Owing to the branchiostegite being open in its ventral — 
aspect, the water in which the crayfish moves has free access - 
to the branchial chambers, so that the gills are continually 
bathed in this element. The amount of water at any one 
time in the branchial chambers is very small, so that the supply — 
must be constantly renewed. The branchiex, being attached to 
certain of the limbs and interarticular parts, are actuated by 
the motions of these limbs, and thus cause a current of fresh 
water to pass over the gills; while to hasten the flow of this 
current a peculiar arrangement exists in the anterior part of 
the branchial chambers. In the channel which leads from 
the interior of these chambers forwards to the outside there 
lies a broad oval-shaped plate, called the ee 
which is attached to the base of the second maxilla. This — 
plate acts as a scoop to bale the water out of the chamber; 
and as the water is baled out, fresh water comes in to supply 
its place, and thus a constant current over the gills is main- 
tained. It is from this water that the oxygen necessary for | 
the renovation of the vitiated blood is obtained. This brings 
us to the consideration of the organs which serve for the 
circulation of the blood. 
The circulatory organs.—In the crayfish, equally as in the 
higher animals, the heart is the driving-wheel in the circula- 
tion of the blood. This organ lies near the posterior end of | 
the thorax, on the tergal aspect of the animal. It is a thick 
muscular body, of an irregular hexagonal shape, lying in a 
comparatively large cavity called the pericardial sinus, to the 
sides of which it is attached by bands of fibrous tissue. The 
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