THE CRAYFISH. 39 



Fio. XIV. Transverse section across the thorax, between the twelfth and thirteenth 

 appendages, after injection as described for Fig. XIII. 



Specially designed to show the great blood passages in relation to the gills. 



On the left side all the parts are drawn in situ, the muscles and gills included. 



On the right the muscles connected with the basal joint of the second ambulatory leg 

 were removed, to fully expose the sternal sinus. 



The gills of the right side are diagrammatic, and no note has been taken of the blood- 

 spaces surrounding the extensor and levator-abdominis muscles. 



The arrows indicate the course taken by the blood in life. 



The central duct of the digestive gland is best seen after preservation in alcohol. <J . x 3. 



The sternal artery is represented in Figs. XIII. and XIV. as passing to opposite sides 

 of the intestine. There is no rule in this matter. 



To complete the study of the blood- vascular system, the circulation should be witnessed 

 under the microscope in some small transparent form. Asellus answers admirably. 



