108 MANUAL OF ANIMAL BIOLOGY 



sternal sinus, and an afferent branchial vessel which carries 

 blood from the gills. In the gill filaments these vessels are con- 

 nected by many tiny capillaries, and as the blood passes through 

 them, the gaseous interchange £akes place. The gill filaments are 

 very thin-walled and are continually bathed in the water which 

 passes through the gill chamber. The blood, with a new supply 

 of oxygen for the tissues, passes from the gills through the afferent 

 branchial vessels, and finally reaches the pericardial sinus from 

 which it passes into the heart and is pumped to all parts of the 

 body. 



4. Excretory System 



The excretory organs of the Crayfish are unusual in their struc- 

 ture and not obviously similar to the nephridia of the Earthworm, 

 although it is generally held that they are homologous with the 

 latter. They consist of a pair of small bodies, the green glands, 

 which are situated in the posterior part of the head, at the base 

 of each antenna. Each green gland consists of three parts : (a) a 

 glandular portion, which takes the wastes from the blood ; (b) a 

 thin-walled, sac-like bladder, which receives the wastes given off 

 by the glandular portion ; and, leading from the bladder, (c) a 

 fine tube which opens to the exterior through the wall of the basal 

 segment of each of the antenna. The green glands are vascular- 

 ized by small branches from the antennary arteries. The blood, 

 which they receive, passes through the glandular portion of the 

 organ, and the nitrogenous wastes are taken from it. (W. f. 63.) 



5. Reproductive System 



The Crayfish is a dioecious animal ; that is, the two sexes are 

 separate. This is different from the hermaphroditic condition 

 which has previously been noted in the Earthworm. 



Male Organs of Reproduction. The sperm are developed in 

 one testis situated near the pericardium. The anterior portion of 

 the testis is bilobed, while the posterior end is single. Leading from 

 the right side of the posterior portion is a duct, the vas deferens, 

 which opens to the exterior through the coxopodite of the fifth 

 walking leg. The vas deferens from the left side of the testis has 

 the same structure. The testis is largely composed of a mass of 

 fine tubules in the walls of which the sperm are formed. When 

 the sperm have reached the proper stage of maturity, they pass 



