SENSORY ORGANS OF THE LOBSTER 181 



and open to the outside by a hair-protected aperture at the 

 distal angle of the membranous wall of the capsule (Fig. 

 74). Inside the capsule is a gelatinous mass of semi-fluid 

 material, through which fine sensory hairs, innervated from a 

 main sensory branch, are abundantly distributed. The otoliths 

 or crystals are also distributed throughout the gelatinous 

 matrix. When the equilibrium of the organism is disturbed 

 these otoliths impinge on the sensory hairs, and thus originate 

 stimuli and motor responses by which the animal regains its bal- 

 ance. Sound vibrations may also have the same effect on the 

 hairs directly. The so-called " auditory " organ perhaps has less 

 to do with sound vibrations than with the balance or equilibrium 



FIG. 76. Centrolecithal type of egg and cleavage in the crayfish. The nuclei, 

 after several divisions, pass to the periphery of the egg after which radial cleavage 

 planes divide it into cells. (From Parker and Haswell.) 



of the body, and compares with the lateral organs of fishes or 

 the semi-circular canals of vertebrates. 



4. The Eyes. The eyes of arthropods are entirely different 

 from those of higher groups of animals. Vision is compound, 

 the eyes being made up of thousands of minute units, termed 

 ommatidia, each ommatidium having a complex structure (Fig. 

 75). The facets (cornea), like mosaics, form the outer cuticle of 

 the eye. 



REPRODUCTIVE SYSTEM. The sexes are separate in the ma- 

 jority of Crustacea, and the gonads are relatively much larger 

 than in the earthworm. The testes are long, beaded organs, 

 white in color, lying dorsally to the hepato-pancreas, one on each 

 side of the dorsal blood system. The male gonoduct or vas 

 deferens originates about two-thirds of the length from the 

 anterior end, and runs downward through the body cavity to 



