106 Descriptions of Preparations. 



35. Common Crayfish (Astacus Fluviatilis), 



Dissected so as to show its nerve system ; the greater part of all the terga of the 

 animal, and the viscera of organic life, with the exception of the commencement 

 and the termination of the dii^estive tract, having been removed. 



Besides the prae-oral or so-called ' supra-oesophag-eaF ganglionic 

 masSj supplying the eyes, antennules and antennae, there are twelve 

 post-oral ganglia in the Crayfish, of which six belong to the seg- 

 ments anterior to the post abdomen, and six to these terminal 

 segments. The first post-oral ganglion is the largest of the series, 

 and supplies no less than six pairs of appendages, viz. the man- 

 dibles, the two pairs of maxillae, and the three pairs of foot-jaws. 

 In the developing Crayfish, as shown by Rathke", this mass which 

 is thus fused in the adult is represented by six pairs of white 

 specks. Posteriorly to it we see five ganglia, remaining distinct, 

 and corresponding in the adult as they do in the embryo of the 

 Macrurous Decapods, and also of the Hedriopthalmatous Crustacea, 

 with the five pairs of abdominal feet. Each of these six ganglionic 

 masses presents a perfectly fused and continuous surface, but traces 

 of the primitive antero-posterior bifidity of the nerve chain are 

 still faintly visible, even to the unassisted eye, in the commissural 

 cords passing between the ganglia. The wide opening which is seen 

 in the commissural cord connecting the third and fourth abdominal 

 ganglia, gives passage to the sternal artery seen in Prep. 32, p. 99, 

 and the cord which connects the fourth with the fifth, differs from 

 the cords which connect the pairs anteriorly placed in being shorter 

 than any one of them. There are six post-abdominal ganglia, under 

 the third and fourth of which, as also under the cord of commissure 

 between the fifth and last, slips of blue paper have been placed. 

 Each of the five anterior post-abdominal ganglia gives ofii" two 

 pairs of nerves, the sixth, which is the largest of the series and 

 alone of the six connected with the one preceding it by a double 

 commissure, gives oflF a considerably larger number of nerves, 

 supplying as it does both the anus and the telson, in addition to 

 the parts homologous with those supplied by the five anterior 



u Ueber die Bildung und Eutwickelung des Flusskrebses, pp. 32, 33. 



