2 10 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. 



ft. 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 



