CRUSTACEA. 37 



divisions (// 3, h 2) are similar in general appearance. 

 Lastly, the eye-stalk (jty) is supposed to represent the 

 basal' section '(// 1), the three appendages borne on this 

 not being developed. 



On the upper side of the basal section of the little 

 antennae are the ears of the lobster (Fig. 8, A, a 1. 

 ear). These organs are detected externally by a small 

 oval space of a clearer aspect than the rest of the sec- 

 tion. This space is protected by hairs, and when these 

 are scraped or cut away an opening is seen on the 

 upper inner edge, through which a bristle maybe passed 

 into the interior. 



If, now, the soft, external membrane covering the oval 

 space is carefully removed, together with the calciticd por- 

 tion of the section and the muscles beneath, a white, 

 semi-transparent sac is seen, in which one end of the 

 inserted bristle will be detected. Opening this sac, it is 

 found to contain very small grains of sand with other 

 foreign matters and a thick fluid. A projecting ridge is 

 also observed, covered with fine hairs, which are con- 

 nected at their bases with brandies of the auditory nerve. 

 A sound-wave sets the sand particles in motion, the vibra- 

 tions affect the hairs, and the impressions of these vibra- 

 tions are conveyed along the auditory nerve to the brain. 



So far, the pupils have discovered, in spite of exces- 

 sive growth and differences of position on the one 

 hand, or of non •development on the other, one plan 

 common to all the appendages. 



The resemblance existing between the appendages 

 arranged along an axis like those of the lobster is 

 known as serial symmetry, and one appendage can be 

 called the serial homotype of another, and the leg of 



