344 CRUSTACEA. 



The compound eyes of Crustaceans have not, however, as yet been 

 examined with the same patient diligence as those of the cock- 

 chaffer ; so that, as relates to their minute anatomy, much is still left 

 to conjecture and uncertainty. One peculiarity connected with 

 these organs is, that in the two highest orders of Crustacea, hence 

 called Podophthalmia, the eyes are placed at the extremity of 

 moveable pedicles articulated with the first cephalic ring of the ex- 

 ternal skeleton, and thus they may be turned in various directions 

 without moving the whole body at the same time. This provision 

 was not required in insects, owing to the mobility of the head in 

 those animals; but is absolutely indispensable in the case before us, 

 where, the head and thorax being consolidated into one mass, the 

 extent of vision commanded by sessile eyes would have been ex- 

 ceedingly limited, and inadequate to the security of creatures ex- 

 posed to such innumerable enemies. 



(380.) It is in the higher Crustacea that we, for the first time, 

 indubitably find a distinct auditory apparatus ; and, from the 

 simplicity which the organ of hearing presents in this its earliest 

 appearance, an inquiry concerning its structure becomes of great 

 physiological interest. In the lobster the ears are situated upon 

 the under surface of the basal joints of the second pair of antennae. 

 On looking carefully in this situation the student will find a pro- 

 minent tubercle formed by the shell, the top of which is perforated 

 by a small circular opening covered with a tense membrane. Be- 

 hind this orifice is placed a minute vesicle filled with fluid, upon 

 which a delicate branch of the antennary nerve is distributed. 

 This constitutes the whole apparatus : the vibration of the water 

 strikes upon the external membrane, the water in the sacculus 

 participates in the tremor, and the expanded nerve conveys to the 

 brain the sensation thus produced. 



In the Brachyura, or crabs, the membrane covering the external 

 orifice of the ear is converted into a moveable calcareous lamella, 

 from which, in some genera, a furcate process is continued inter- 

 nally ; so that the whole, when removed by maceration, has no 

 very distant resemblance to the stapes of the human ear, and, like 

 it, seems to be acted upon by muscular fasciculi, so disposed as to 

 regulate the tension of the vibratile membrane, and thus adapt it to 

 receive impressions of variable intensity. 



(381.) One of the first circumstances calculated to attract the 

 notice of the anatomist who turns his attention to the structure of 

 the generative system both in male and female Crustacea, is the 



