ASTACUS FLUVIATILI- 



renewal of the water with wiiich they are in contact. This 

 object is attained principally by the action of the scapho- 

 gnathitr, which lies immediately behind the anterior opening 

 of the branchial chamber; and, dimiiir lite, is incessantly in 

 motion, billing out, as it were, the water which \\;i> b0QOBM 

 impure through the anterior opening, and thus compelling 

 the flow of fresh fluid into the branchial chamber through iis 

 posterior and inferior opening, constituted by the space left 

 between the lower edge of the branchiostegite and the bases 

 of the limbs. 



Tin* nervous system of Astactts 1 is composed of thirteen 

 principal ganglionic masses, of which one, cerebral, lie> in 

 the head, in front of the mouth ; six, thoracic, are situated 

 in the sternal canal ; and six, abdominal, lie in the median 

 sternal region of the six anterior somites of the abdomen. 



The cerebral ganglia (Fig. 74, b ; Fig. 75, a) give off 

 nerves to the eyes and to the muscles of the ophthalmic 

 appendages ; to the antennules and the auditory organs 

 which they contain ; to the antennae and the sac of the 

 antennary gland ; to the carapace in front of the cervical 

 suture ; and, finally, they send posteriorly two long and 



Pio. 75. Visceral nerves of Astacus. a. cerebral ganglia ; b. commissures that of 

 the right side is cut and turned back: c. ir.in-v, rs<- mnl unitini; them behind 

 the oesophagus, (E : d, </, rf, axy : A, ganglion: i, lateral branch <>r 



azygos, uniting with postero-latcral nerve g ; f. ant-r > lat -r.-ii nerve ; f. mcdio- 

 lateral nerve; *, hepatic nerve ; P, pylorlc; C, cardiac po.tion of stomach. 



stout commissural cords to the anterior thoracic ganglionic 

 mass. These commissures are connected by a transverse 



1 For the histology of the nervous system, w an elaborate essay by 

 Haeckel on tin- minute structure of the tissues of the Crayfish, in the * Archiv 

 fur Anatonm-/' L( 



