100 Descrijytions of Preparations. 



besides the straight intestine, the nerve cord, and the post-abdo- 

 minal artery ah-eady mentioned, the powerful and complex flexor 

 muscles of the post-abdominal segments and the ' svvimmeret ; ' 

 fascicles from which are observable passing to find attachment 

 to 'apodemata/ as far forwards as the entrance of the oesophagus. 

 It is by these muscles that the more rapid movements of these 

 animals are executed. For their slower crawling movements, the 

 ambulatory legs are employed, and the muscles which act upon 

 the limbs of the right side, may be seen passing upwards through 

 the apodematal cells to take origin from the epimera as seen in 

 Prep. 34. Some muscular fasciculi not ordinarily described pass 

 backwards from the sternal regions, anteriorly to the mouth on to 

 the anterior surface of the stomach, decussating thus with the 

 commissural and stomato-gastric nerve cords. 



As a sexual pecuharity, the modifications of the first and second 

 post-abdominal appendages should be noted. But the testis itself 

 does not extend further backwards than the posterior border of the 

 carapace, and the outlet of the vas deferens, as being placed in the 

 basal joint of the last ambulatory leg, justifies the application of 

 the term ^post-abdomen' to the segments placed posteriorly to it. 



For figures of the Crayfish as thus dissected and described, see 



plate vii. 



For a detailed account of the flexor muscles of the post-abdominal 

 segments, see Milne-Edwards, ' Suites k Buffbn,' Histoire Na- 

 turelle des Crustaces, torn, i. p. 157> tom. iii. pi. xiii. 



For the late period at which the digestive tract is developed in 

 Arthropoda, and the possibility of thus accounting for its 

 comparative simplicity and the straightness of its course, see 

 Zaddach, Untersuchungen iiber die Entwickelung und den 

 Bau der Gliederthiere, p. 42. 



33. Common Crayfish {Astacus Fluviatilis), 



Dissected so as to show the heart, its six arteries, and two of its six venous inlets, 



in situ. 



The cardiac and part of the cephalo-stegal portion of the carapace 

 has been removed, as well as a flattened venous sinus which was 



