PHYLUM ARTHROPODA 



211 



ommatidia (Fig. 118, B). When this occurs 

 the ommatidia no longer act separately, but 

 a continuous image is thrown on the retinu- 

 lar layer. This is called a superposition im- 

 age, which is much less distinct than the 

 apposition image, but it is more sensitive to 

 weak intensities of light. 



Statocysts. The statocysts of Cambarus 

 are chitin-lined sacs situated one in the 

 basal segment of each antennule. In the base 

 of the statocyst is a ridge with many fine 

 sensory hairs which are innervated by a sin- 

 gle nerve fiber. Among these hairs are a 

 number of large grains of sand, the stato- 

 liths, which are placed there by the crayfish. 

 Beneath the sensory cushion are glands 

 which secrete a substance for the attachment 

 of the statoliths to the hairs. 



The statocyst for many years was con- 

 sidered an auditory organ, but later investi- 

 gations have proved that it is an organ of 

 equilibrium. The contact of the statoliths 

 with the statocyst hairs determines the orien- 

 tation of the body while swimming, since 

 any change in the position of the animal 

 causes a change in the position of the stato- 

 hths under the influence of gravity. When 

 the crayfish changes its exoskeleton in the 

 process of molting, the statocyst is also shed. 

 Individuals that have just molted, or have 

 had their statocysts removed, lose much of 

 their powers of orientation. Perhaps the 

 most convincing proof of the function of 

 equilibrium is that furnished by experi- 

 ments. Shrimps, which had just molted and 

 were therefore without statoliths, were 

 placed in filtered water. When supplied with 

 iron filings, the animals filled their stato- 

 cysts with them. A strong electromagnet was 

 then held near the statocyst, and the shrimp 

 took up a position corresponding to the 

 resultant of the two pulls, that of gravity 

 and that of the magnet. 



Endocrine glands 



The sinus gland located in the base of the 

 eye stalk produces two hormones, and pos- 

 sibly more. These hormones appear to con- 



trol the spread of pigment granules in the 

 chromatophores in the compound eyes and 

 in the body epidermis. They govern to a 

 greater or lesser extent, metabolic rate, 

 growth and viability. They also regulate the 

 frequency of molting, and are necessary for 

 normal deposition of calcium salts in the 

 exoskeleton. These hormones are distributed 

 by the blood stream, as in the vertebrates. 

 It has been observed in experimental ani- 

 mals that removal of the sinus glands 

 shortened the life of crustaceans. 



Muscular system 



In the crayfish the complex muscles are 

 all attached to the inner surface of the skele- 

 ton, instead of constituting a part of the 

 body wall as in the coelenterates and anne- 

 lids, or being external to the skeleton as in 

 man. The largest muscles in the body of the 

 crayfish are situated in the abdomen (Fig. 

 114) and are used to bend that part of the 

 animal forward upon the ventral surface of 

 the thorax, thus producing backward loco- 

 motion in swimming. Other muscles extend 

 the abdomen in preparation for another 

 stroke. Muscles of considerable size are 

 situated in the thorax and within the tubu- 

 lar appendages, especially the chclipcds. A 

 comparison of the skeleton and muscles of 

 the crayfish with those of man is interesting. 



Reproductive system 



Sexes are normally separate in the crayfish, 

 there being only a few cases on record where 

 both male and female reproductive organs 

 were found in a single specimen. 



Male reproductive organs. The male or- 

 gans consist of two white testes partially 

 fused into three lobes and on each side a 

 long coiled sperm duct, the vas deferens, 

 which opens through the base of the fifth 

 walking leg. The testes lie just beneath the 

 heart (Fig. 114) . They constitute a soft body 

 possessing two anterior lobes and a median 

 posterior extension. 



Spermatogenesis. The primiti\'e germ 

 cells within the testis pass through two 



