300 ^WE ANIMAL KINGDOM 



In the crayfish a dorsal part of this cavity is separated off by a 

 partition, the pericardial membrane, to form a pericardial sinus around 

 the heart. When the heart contracts blood is pumped anteriorly, pos- 

 teriorly and ventrally through arteries that branch out to all parts of 

 the body. Eventually the arteries end, and the blood is poured into the 

 hemocoel. It then drains ventrally into the perineural sinus from which 

 veins carry it to the gills. After passing through capillaries in the gills 

 the blood continues in veins toward the heart, and is emptied into 

 the pericardial sinus. It enters the heart during relaxation through 

 slitlike valves in its sides. The blood is nearly colorless, but becomes 

 bluish when exposed to air because of the presence of the oxygen- 

 carrying pigment hemocyanin, a copper-containing protein. Hemo- 

 cyanin is also found in some arachnids and molluscs. 



The excretory system of the crayfish is the green glands at the 

 base of the second antennae. Each consists of a ventral green glandular 

 part bathed in blood and a dorsal bladder. Wastes removed from the 

 blood in the glandular part pass through ducts and are stored in the 

 bladder. A duct from the bladder opens on the ventral surface of the 

 basal antennal segment. 



Paired gonads lie beside the midgut and fuse together over it 

 (Fig. 16.8 A). In the female a straight oviduct passes ventrally on each 

 side to open on the basal segment of the middle pereiopods (segment 

 12). In the male a pair of sperm ducts follow a similar but convoluted 

 course, opening on the basal segment of the last pereiopods (segment 14). 

 The sperm are peculiar in lacking flagella and are gathered into bun- 

 dles or spermatophores by secretions of the ducts. 



At copulation the male turns the female on her back, holding her 

 with pincers and other chelate pereiopods. The first pleopods, which 

 otherwise lie forward against the body between the bases of the pereio- 

 pods, are then depressed against the female. Spermatophores issuing 

 on the last pereiopods pass down grooves on the modified pleopods to 

 the female, where they adhere tightly between the bases of the posterior 

 pereiopods. In the lobster and in some crayfishes the females have a 

 small hollow, the seminal receptacle, between the bases of the fourth 

 and fifth pereiopods where spermatophores are fastened. 



Some days or weeks later the eggs are laid. The female lies on her 

 back with the abdomen folded tightly against the thorax. As the eggs 

 emerge they are fertilized and glued to the pleopods. They hatch after 

 several weeks into miniature crayfish that remain attached for a while 

 to the mother. 



1 37. Daphnia, the Water-Flea 



The crayfish is a good example of a large crustacean, but many of 

 this class are small and reduced in their complexity. The water-fleas (or- 

 der Cladocera), 1 to 3 mm. long (Fig. 16.10), are described here as an ex- 

 ample of small crustaceans because they are transparent and can be 

 studied easily without dissection. They live primarily in open fresh 

 water as part of the plankton. The genus Daphnia is represented all 



