210 ELEMENTARY BIOLOGY. [CHAP. 



a. The gastrolith ; a discoidal stony-mass, interposed 

 between the cellular and cuticular layers of the 

 anterior cardiac wall. 



This is developed only in the summer season ; an 

 underlying thickening of the chitinous cuticle is in- 

 variably present to mark its position. 



b. The postero-ventral ossicles ; delicate vertically elon- 

 gated bars in the hind wall of the cardiac sac. 



c. The accessory lateral ossicle; a small bar, extending 

 backwards and downwards from the point of junc- 

 tion between the two main lateral ones. 



d. The intestine, the orifice of the digestive duct and 

 the coecum. (Cf. supra.} 



4. Reduce the whole gut to the condition of median 

 longitudinal section, by carefully removing the exposed 

 half with scissors ; wash until quite clean and examine 

 under water. 



a. The gullet ; a spacious tube placing the stomach in 

 communication with the exterior. Note its re- 

 lations to the mouth. 



b. The median cardio-pyloric valve ; seen, in section, 

 as an upgrowth constricting the passage from the 

 cardiac to the pyloric chambers. 



c. Follow the cut edge of. the chitinous lining of the 

 stomach ; note 



a. That it is continuous at the oral aperture with 

 the exoskeleton. 



f$. That it ends abruptly, in front of the orifice of 

 the digestive duct; terminating in a series of 

 valve-like processes, one of which can be very 



