GENERAL ZOOLOGY 



The circulatory system of the crayfish, unhke that of such anneUds as the 

 earthworm, is not a "closed" system with capillaries. Rather, it resembles 

 superficially the "open" circulatory system described for most mollusks. 

 Branching arterial vessels conduct blood from the heart to the organs, where 

 the arteries terminate (Fig. 15.5). Leaving the arteries, blood percolates 

 through the tissues and collects in sinuses, or hemocoelic spaces. From the 

 general hemocoel blood flows ventrally into the sternal sinus and the lateral 

 sinuses with which the sternal sinus communicates. From the lateral sinuses 

 a series of vessels constituting the branchial circulation carries blood through 

 the gills for oxygenation. The blood then passes through a system of venous 

 channels into the pericardial sinus surrounding the heart, and re-enters the 

 heart, when it relaxes between beats, through three pairs of valved openings, 

 the ostia. Such an "open" system is not markedly efficient; the pressure 

 exerted by the heart in contraction is rapidly dissipated in the large sinuses, 

 and blood flow through the gills is relatively slow. 



Such a system obviously suffices for the needs of the animal, however. 

 Blood flowing to the tissues transports to them food pickied up in the digestive 

 glands, and oxygen from the gills, and carries away carbon dioxide and 

 nitrogenous wastes. Carbon dioxide is exchanged for oxygen in the branchial 

 circulation, and nitrogenous wastes are removed from the blood in the ex- 

 cretory organs. 



The gills and their circulation, which constitute the mechanism of gas 

 exchange, have been adequately described. The efficiency of this mechanism 

 is enhanced by the maintenance of an external water circulation over the gills. 

 In the living crayfish water is continually drawn under the posterior and 

 ventral edges of the carapace into the gill chambers, where it passes forward, 

 bathing the gills, and is ejected anteriorly. These water currents are pro- 

 duced by a specialized, paddle-like extension of the second maxilla on each 

 side of the body. As previously mentioned, the swimmerets aid by keeping 

 the water in motion about the posterior, ventral part of the body. 



The excretory organs are a pair of compound tubular structures termed the 

 green glands. These lie in the hemocoel, one on each side, just anterior to 

 the stomach. Each (Fig. 15.6) consists of a terminal, flattened end sac, a 

 convoluted labyrinth or cortex, and a distal tubular portion, at least partially 

 lined by secretory epithelium, leading from the labyrinth to a small bladder. 

 The bladder opens externally by a nephridiopore on the basal segment of the 

 antenna. The green gland is interpreted as a compound nephridium; the end 

 sac is considered the remnant of a coelomic pouch. The blood supply to these 

 organs is copious, derived from branches of the lateral cephalic and ventral 

 thoracic arteries. The functions of the green glands are not completely under- 

 stood. According to current interpretations, nitrogenous wastes and salts are 

 extracted from the blood in the labyrinth and proximal parts of the tubule; 

 as the urine thus formed flows outward through the tubule, secretory cells 

 lining the distal portion selectively reabsorb salts and return them to the 

 blood. The comparatively watery urine remaining in the tubule is then 



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