98 Descriptions of Preparations. 



For generalizations as to the homologies of the appendag-es in the 

 various orders of Crustacea, see Spence Bate, ' History of British 

 Sessile-eyed Crustacea/ Spence Bate and Westwood, 1868, 

 Introduction, pp. vii-xx. 



For the bibliography of memoirs upon the anatomy of the Fresh- 

 water Crayfish, see Brandt and Ratzeburg, ^ Medizinische 

 Zoologie,' Bd. ii., p. 6^, note i ; or, Leydig, ' Handbuch der 

 Vergieichendeu Anatomic/ p. 353. 



For the differences between Macrurous and Braehyurous Crustacea, 

 see Dana, ' Crustacea, U. S. Exploring Expedition/ pt. i., p. 49. 



32. Common Crayfish {Astacus Fluviatilis), 

 Male, 



Dissected so as to show the nervous, circulatory, and digestive systems in situ, and in 

 the relations they hold to each other and to the external tegumentary skeletal 

 system. 



The nerve system underlies the sternal elements of the various 

 segments ; the heart, the dorsal portion of the omo-stegite, giving 

 off both anteriorly and posteriorly an azygos artery which may 

 represent the more elongated and vasiform heart, which is more 

 usual in Arthropoda than the compressed irregularly polygonal 

 organ seen here ; and the digestive tract occupies a position be- 

 tween those of the two other systems. A white bristle has been 

 introduced through the mouth into the oesophagus and stomach, 

 and shows that this latter organ is prolonged as far forwards in- 

 ternally as the antennary sternum is externally, and to a point, 

 therefore, much in front of the plane of entrance of tlie oesophagus. 

 In order to show these points, the external skeleton and its append- 

 ages, with the exception of the eyes, antennule, and antenna, have 

 been removed on the animal's left side, together with the muscles 

 in connection with them, and together with the two lobes of the 

 liver and the ' green' or ' antennary' gland of the same side. A slip 

 of blue paper has been placed under the nerve cord in the post- 

 abdominal region, in the interval between its penultimate and ante- 

 penultimate ganglia ; a second has been placed in the same region, 

 but in its dorsal portion, under the azygos caudal artery and above 



