ANNU LOS A CRUSTACEA. 7 3 



of the legs, and enclosed in a cavity formed beneath 

 the carapace on each side of the body. These cavities 

 are furnished with two openings, and circulation of 

 water is kept up within them by the movements of the 

 legs. In some of the lower groups, there are no dis- 

 tinct respiratory organs. 



96. The nervous system is similar to that found in 

 the Insecta. There is a pair of ganglia developed in 

 each somite. These are connected by double nervous 

 cord-5. The cephalic ganglia are placed above the 

 oesophagus, the others run along the ventral surface 

 of the body, the gullet passing between the two front 

 pairs. 



The higher Crustacea possess compound eyes, similar 

 to those of the insects. They are often placed at the 

 summit of movable stalks, which are sometimes of con- 

 siderable length. In the lower forms, the eyes are 

 simple. There is often only one eye, which is placed in the 

 middle of the head. 



97. The organs of hearing in the higher groups are placed 

 close to the base of the long antennae. They consist each 

 of a hollow sac which is closed externally by a thin mem- 

 brane. Behind this is a cavity filled with fluid, which 

 is connected with the auditory nerve. 



98. Development. The Crustacea are oviparous animals, 

 and, except one group, are all unisexual. The eggs, 

 after exclusion, are generally carried about by the mother 

 until near the period of hatching. They are either at- 

 tached to the abdominal feet, or placed in pouches on 

 each side of the tail. The young animals are often so 

 unlike the adult, that, until the connection was observed, 

 they were believed to be of different species. The early 

 stages of the common shore-crab are thus described by 

 Gosse : " A hemispherical carapace or shell, not so big 

 as a small pin's head, sends up from its centre a long 

 pointed, curved spine, while another spine curves down- 

 wards from the front beneath the body like a beak ; the 

 eyes are without stalks; there are two pairs of jointed 



