COMMON CRA YFISH. 307 



PLATE X. 



COMMON CRAYFISH (Astacus fluviatilis), MALE, 



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

 various organs having been exposed in a vertical section, by the removal of the tegumentary skele- 

 ton, the muscles, and the hepatic lobes of the left side. See Prep. 33, p. 177, ante, 



a. OESOPHAGUS leading vertically upwards from the mouth into the 



stomach. The labrum, the free edges of the mandibles, and of the 

 two maxillae, are faintly indicated on the right side of the mouth 

 anteriorly to the three foot-jaws. 



b. Cardiac portion of stomach. Superiorly and anteriorly the stomach is 



still retained in its natural position, the anterior gastric muscles, which 

 took origin superiorly to' the supra-oesophageal ganglia, o, from the 

 under surface of the ventral wall of the hollow rostrum, and attached 

 themselves to the cardiac plate, having been left intact ; whilst the 

 anterior wall of the stomach has been displaced a little backwards in 

 order to give a better view of the stomato-gastric nerves. 



c. Lateral prominence of pyloric filter. 



d. Hepatic lobes of right side. 



e. Orifice by which the hepatic lobes of the left side opened into the 



digestive tract. 



f. Intestine passing with the straight course characteristic of Crustacea, 



with the exception of Lynceus, to the anus. 



g. Anus, opening on the inferior surface of the ' telson ' in uncalcified 



membrane, anteriorly to the junction of its anterior and posterior 



halves. 

 //. Heart, showing one of the lateral venous orifices, and the posterior 



aorta which divides into two main branches, i and/. 

 i. Post-abdominal artery, taking a course superiorly to the intestine, and 



inferiorly to the extensor muscles of the posterior somites. 

 j. Sternal artery passing down to the interval between the commissural 



cords connecting the third and the fourth abdominal ganglia. 

 k. Hepatic artery of the left side, passing from the heart over the pylorus 



towards the left hepatic lobes which have been removed. 

 /. Anterior left lobe of testis. 



t. Azygos lobe of testis placed behind the paired lobes. 

 m. Convolutions of vas deferens of left side, in length equal to that of the 



entire body. The agglutinating matter of the spermatophores is 



probably secreted in them. 

 n. Coecal sac of mesenteron or archenteron. 

 o. Supra-oesophageal ganglionic mass. 



X 2 



