ASTACUS FLUVIATILIS. 



285 



renewal of the water with which thej are in contact. This 

 object is attained principally by the action of the scapho- 

 gnathite, which lies immediately behind the anterior opening 

 of the branchial chamber ; and, during life, is incessantly in 

 motion, baling out, as it were, the water which has become 

 impure through the anterior opening, and thus compelling 

 the flow of fresh fluid into the branchial chamber through its 

 posterior and inferior opening, constituted by the space left 

 between the lower edge of the branchiostegite and the bases 

 of the limbs. 



The nervous system of Astacus^ is composed of thirteen 

 principal ganglionic masses, of which one, cerebral, lies 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 antennas and the sac of the 

 antennary gland ; to the carapace in front of the cervical 

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



Fig. 75.— Visceral nerves of Astacus.—a, cerel)ral £ranfrlia ; b, commi^snres— that of 

 the right Bide is cut and turned back ; c, transverse cord unitini^ them behind 

 the oesophagus, (E ; d, rf, d, azyiros nerve ; h, ganglion; i, lateral branch of 

 azygos, uniting with postero-lateral nerve g ; e, antero-latara! nerve ; f. medio- 

 lateral nerve; k, hepatic nerve ; P, pyloric; C, cardiac portion of stomach. 



stout commissural cords to the anterior thoracic ganglionic 

 mass. These commissures are connected by a transverse 



* For the histology of the nervous system, see an elaborate essay by 

 Haeckel on the minute structure of the tissues of the Crayfish in the ' Archiv 

 fiir Anatomie," 1857. 



