COMMON CRAYFISH. 187 



36. COMMON CRAYFISH (Astacus fluviatilis), MALE, 



Dissected so as to show its nervous system. 



THE supra-oesophageal ganglion and the twelve post-oral ganglia 

 of the adult Crayfish, of which six belong to the thorax and six to the 

 abdomen, have been displayed by the removal of the whole tergal region 

 of the body, of the viscera of organic life, and the endophragmal skeleton 

 in the thorax. The oesophagus, through which a black bristle has been 

 passed, and a small terminal portion of the intestine remain in situ. 



The supra-oesophageal ganglion was seen by Rathke to be made up 

 of two rudiments in the embryo ; of which the posterior, or the one placed 

 nearest to the mouth, was the larger and supplied the first and second 

 antennae. The ganglion itself in the adult gives off nerves to the eye, 

 the eye-muscles, to the integument of the head, and the first and second 

 antennae, besides furnishing two azygos nerves, one anterior, the other 

 posterior, to the stomato-gastric nerve. The superior and inferior roots 

 of this nerve (infra), together with the two commissures to the sub- 

 oesophageal or first post-oral ganglion, are seen passing over a piece of 

 blue paper placed under them in front of the oesophagus. The left end 

 of this blue paper rests on the secreting portion of the left antennary or 

 green gland. The infra-oesophageal ganglion is the largest of the post-oral 

 series and innervates no less than six pairs of appendages, viz. the 

 mandibles, the two pairs of maxillae, and the three pairs of maxillipeds 

 or foot-jaws. In the developing Crayfish, as shown by Rathke, this mass 

 is represented by six pairs of white specks. It is followed by five thoracic 

 ganglia, which remain distinct and correspond in the adult as well as in the 

 embryo of Macrurous Decapods to the five pairs of thoracic feet. Each 

 ganglion is connected to its successor by two longitudinal commissures, 

 showing the primitive bilateral composition of the chain. The commissures 

 between the third and fourth ganglia are widely separate for the passage 

 of the sternal artery seen in Preparation 33. The fourth and fifth 

 ganglia are approximated. The first abdominal ganglion is some distance 

 behind the last thoracic, and all the six abdominal ganglia are equi- 

 distant one from the other. A slip of blue paper has been placed under 

 the third and fourth, and another under the commissure to the last of 

 the series. The commissural cords are clearly double. The third ganglion 

 is seen to give off a pair of nerves on each side, while another pair springs 

 from the commissures immediately behind the ganglion. The anterior 

 nerve on each side goes to the swimmerets ; the posterior and the com- 

 missural pair to the muscles of the same, i. e. the third somite. All 

 the abdominal ganglia resemble the third pair in these points with the 



