132 COMPARATIVE ANATOMY 



and it is in constant motion, ladling the water out through 

 the anterior opening. The edges of the branchiostegite being 

 free, the branchial chamber is open both below and behind — 

 the former passage is largely blocked by the bases of the 

 limbs, but the latter is unobstructed, and water flows freely in 

 through it to replace that baled out by the scaphognathite. 

 The arthrobranchiffi and functional pleurobranchia are plume- 

 like structures, consisting of an axis or stem to the sides of 

 which a number of delicate branchial filaments are attached. 

 The cavity of the axis is divided by a longitudinal partition 

 into an outer and an inner blood channel, from which lateral 

 channels are given off to the filaments. The podobranchias 

 have a somewhat different structure, the stem arising from a 

 broad basal piece covered with setse and attached to the 

 coxopodite, while its distal part is modified in that its inner 

 and posterior face is produced into a wide membranous lamina, 

 plaited longitudinally and folded once upon itself longitudinally 

 in such a manner that the edge of the fold looks forward 

 (fig. 27, VII). The anterior and outer faces of the stem are 

 covered with filaments like those of the arthrobranchiaa. 



The blood in the ventral sinus, rich in carbonic acid after 

 its passage through the tissues, passes up the outer channels 

 of the axes of the gills, circulates in the gill filaments, whose 

 thin walls admit of a free interchange of the gases dissolved 

 in the blood and the water, and having got rid of its excess of 

 carbonic acid and taken up oxygen, is returned down the 

 inner blood channels of the axes into the branchio-cardiac 

 sinuses, whence it is conveyed to the pericardial sinus, and 

 thence is admitted into the heart by the ostia. 



The excretory organs of the adult crayfish consist of a pair 

 of "green glands" situated in the head and opening to the 

 exterior on the hinder faces of the tubercles on the basal joints 

 of the second antennse, in the same position, be it noted, as 

 the provisional excretory organs of the larval Apus. A "green 

 gland" consists of a duct, a bladder, and a green glandular 

 mass, from which it derives its name. The duct is a short 

 tube lying in the coxopodite of the second antenna, behind 

 which it turns sharply backwards, and enlarges to form a con- 

 siderable membranous sac with thin walls, lying in front and 

 to the side of the oesophagus. The glandular mass lies below 

 the bladder, close against the sternite of the second antenna, 



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