II.] THE CRAYFISH AND LOBSTER. 209 



e. The antero- median (urocardiac) ossicle; a long 

 tongue-shaped bar, passing back, from the antero- 

 dorsal one, in the middle line of the roof of the 

 cardiac chamber. 



/ The postero-7nedia7i (prepyloric) ; a similar but 

 shorter bar, arising from the postero-dorsal piece. 

 When the parts are at rest, it passes obliquely down- 

 wards and backwards, meeting the antero-median 

 under cover of the postero-dorsal. 



It will be observed that the above-named parts 

 form a repetitional series ; the roof and side walls 

 of each chamber are calcified to form a dorsal trans- 

 verse, a median, and a pair of lateral ossicles, the 

 two latter sets articulating with each other. 



g. The digestive gland; a paired mass, roughly trilobed, 

 on either side. It is an aggregation of short 

 coeca. 



Carefully dissect away its upper half on one side, 

 until a spacious ce7itral duct is reached, around 

 which the glandular cceca are seen to be arranged. 

 Wash until quite clean, and look for their orifices. 



Follow the duct inwardly; it enters the head of 

 the intestine by a wide aperture, immediately in 

 front of the ccecum (i. ^). 



3. Place the animal on its side and remove the exposed 

 half of the body and the digestive gland, so as fully 

 to lay bare the entire alimentary canal. 



The cellular wall and certain muscles will be seen 

 as before; remove these, and examine the chitinous 

 lining and its associated structures. Note, in addition 

 to the parts described above, the following. 



M. 14 



