THE PHYLA ARTHROPODA AND ONYCHOPHORA 



where appropriate reflex motor acts are initiated to maintain the animal's 

 normal posture. 



The chief light receptors of the crayfish are the paired eyes; each eye is 

 borne on a movable stalk arising from the head. The eyes are of the 

 compound type, made up of large numbers of individual units, the ommatidia 

 (Fig. 15.8). Each ommatidium consists of a distal corneal structure, a lens- 

 like refractile body, migratory pigment cells to adjust the intensity of light, 

 and light-sensitive receptor cells which send nerve fibers toward the brain. 

 Each ommatidium is thus a complete visual unit, capable of responding to 

 its visual field. It is impossible to determine, however, whether the crayfish 

 actually "sees" with its brain a mosaic of individual images or a composite 

 single image. The compound eyes, from their structure, are well adapted for 

 the detection of movement across the visual field, and this may be more 

 significant in the life of the crayfish than actual image formation. 



In the crayfish, as in other Metazoa, the function of the nervous system 

 involves chiefly the short-term, rapid integration of the activities of the ani- 

 mal, or what might be termed nervous coordination. In recent years an 

 increasing body of experimental evidence indicates that chemical or hormonal 

 coordination also plays an important part in regulating the vital functions of 

 the crustaceans generally. Certain glandular structures, notably the eyestalk 

 glands and the so-called "Y-organs," in addition to groups of specialized 

 neurosecretory cells in the central nervous system, produce specific chemical 

 mediators, or hormones. These substances, released into the circulating 

 blood, either directly or indirectly exert profound eflfects on the slower, 

 cyclical changes in the body of the animal. Chemical coordination is in- 

 volved in controlling the reversible color changes in the epidermis, in regu- 

 lating the complex phenomena of the molting cycle, and in controlling 

 various reproductive and metabolic cycles. The chemical mediators in 

 crustaceans are analogous to the hormones produced in the endocrine glands 

 of vertebrates (p. 117) and are apparentlv similar to the hormones of insects 

 (p. 456). 



The Reproductive System, Reproduction, and Development. Crayfishes 

 are dioecious; males and females may be distinguished externally by the 

 structure of the first two pairs of swimmerets. Internally, the reproductive 

 system consists of tubular gonads, ovaries or testes, which are paired anteriorly 

 but unite below the heart to form a median, unpaired posterior lobe. The 

 gonads lie in the thorax, dorsolateral to the digestive tract; the posterior lobe 

 may extend into the first abdominal segment. The reproductive ducts of both 

 sexes are paired, passing around the thoracic viscera laterally and ventrally 

 to separate openings on the basal segments of specific thoracic appendages. 

 The external openings of the oviducts lie on the third pair of pereiopods, those 

 of the ductus deferentes (vasa deferentia) on the fifth pair. During copulation 

 spermatophores, packets of spermatozoa bound together by mucous secretions, 

 are emitted from the male genital openings. These packets are then trans- 

 ferred, by means of the modified swimmerets of the male, into the seminal 



435 



