Chap. XII.] 



THE CRAYFISH. 



243 



To facilitate his study of the ossicles of the gastric mill, the 

 student should remove the stomach with a portion of the 

 oasophagus and intestine, and should boil it in a dilute solution 

 of caustic potash to remove the softer parts. Making a ventral 

 incision along its whole length, he should then pin it out under 

 water, so as to expose the dorsal surface. Fig. 76 A. is a diagram 

 of the ossicles thus seen ; B. shows the ossicles separated in the 

 mid-line, and seen from the ventral aspect ; C. is a longitudinal 

 section of the whole stomach. 



M.G. 



H.G. 



OPS 



F.G 



FIG. 76. CRAYFISH STOMACH. 



A. Gastric ossicles from above. B. The same from below, separated trans- 

 versely. C. The stomach in longitudinal section. 



c. Cardiac ossicle, cce. Caecum, g. Mark of gastrolith. h. d. Duct 

 of digestive gland. F. a. Fore-gut. H. G. Hind-gut. M. a. Mid-gut. 

 ces. (Esophagus, p. Pyloric ossicle, p. c. Pterocardiac ossicle, p. p. Pre- 

 pyloric ossicle, u. c. Urocardiac ossicle, z. c. Zygocardiac ossicle. 



The essential parts of the gastric mill are : 



1. (a.) An anterior transverse plate, the cardiac ossicle (c.), 



the hinder part of which (shaded) is white and 



calcareous, the part in front (dotted) transparent. 



(b.) A smaller posterior transverse plate, the pyloric 



ossicle (p.), the greater part of which is transparent. 



