ARTHROPODA. 97 



Circulation of the blood is effected by the rhythmical contrac- 

 tion of the heart, which acts as a central pump. The valves allow 

 the blood to enter the heart, but prevent it from passing out 

 again into the pericardial sinus during contraction. 



Course of the Circulation. The gills are placed in the course of 

 the blood-current returning to the heart, this passing from the 

 internal gill-channels to the branchio-cardiac canals, and thence 

 into the pericardial sinus. The heart in its diastole, i.e., when 

 expanding, sucks in this purified blood through its ostia, and then 

 undergoing systole (contracting) forces it into the arteries. From 

 these it passes into venous spaces, and ultimately reaches the 

 sternal sinus, loaded with carbon dioxide, and poor in oxygen. 

 From the sternal sinus it enters the external gill-channels. A 

 heart, which, like that of the crayfish, distributes oxygenated 

 blood to the body at large is said to be systemic. 



6. Special respiratory organs are present on the thorax in 

 the form of gills (branchiae), adapted for breathing the oxygen 

 dissolved in water. These are vascular outgrowths covered by a 

 very delicate cuticle. Those on each side are contained in a gill- 

 chamber covered over by the branchiostegite, and communicating 

 with the exterior by a narrow slit above the bases of the thoracic 

 limbs. Parasites are particularly fond of fixing upon gills, since 

 these are always delicate structures with an abundant supply of 

 blood and fresh water, and, besides, are usually well sheltered. In 

 this case such unwelcome guests, as well as particles of sand, &c., 

 are largely prevented from entering by long coxopoditic setce pro- 

 jecting from the basal joints of the third foot-jaws, forceps, and 

 first three walking-legs. Each chamber contains 18 perfect, and 

 2 or 3 rudimentary gills. The free ends of all converge upwards. 

 Six of the perfect gills, podobranchice (Fig. 27, G, H, I, ep), are 

 constituted by the lamellar epipodites of the second and third 

 foot-jaws, forceps, and first three walking-legs, which bear a large 

 number of delicate gill-filaments on their anterior arid external 

 surfaces. Eleven more, arthobranchice (Fig. 27, N), are attached 

 to the membranous junctions between the bases of the foregoing 

 appendages and the body. Two are placed on each of these 

 junctions except the first, which possesses only one. These gills 

 are plume-like, with a central stem bearing numerous gill- 

 filaments. The remaining perfect gill, pleurobranchia, similar in 

 structure to one of the arthrobranchise, is attached to the epimeron 



