INVERTEBRATA. 311 



and pyloric ossicles nearer one another, and so cause the 

 teeth to diverge. More powerful anterior and posterior 

 extrinsic muscles, running from cardiac and pyloric ossicles 

 respectively to the tergum of the carapace, pull those 

 ossicles away from one another and cause the three teeth 

 to converge. 



The main division of the gizzard, in which these ossicles 

 occur, is called the cardiac * chamber. The hinder division 

 is called the pyloric chamber, but it is hardly a chamber, as 

 the walls are thickened and bulge in, reducing its cavity to 

 a tri-radiate slit. The front end of this slit is fringed by 

 a series of long setae, which form a lattice-work strainer, 

 preventing any insufficiently comminuted food from passing 

 into the mesenteron. 



It has already been mentioned that the whole of the 

 cuticle lining the gizzard (ossicles and teeth included) is got 

 rid of at every moult. As the moulting-time approaches, 

 there are developed in the side-walls of the gizzard a pair 

 of white, button-shaped masses of calcium carbonate called 

 the gastroliths. These have nothing to do with the grinding- 

 up of food, but are simply a reserve-storage of calcareous 

 material. Just before the moult they are set free into the 

 gizzard and ground up, and their material is absorbed into 

 the blood, to be presently used in the secretion of the 

 calcified parts of the new cuticle. 



14. Mesenteron and Digestive Glands. The mesenteron 

 is a small chamber, with a median caecum projecting up 

 dorsally (fig. 155). Laterally there arise from it a pair of 

 much-branched tubes, constituting the " liver " or digestive 

 gland. These are the largest organs in the body, occupying 

 an astonishing amount of space, and at first sight, in dis- 

 secting, look compact ; but they are soon found to be merely 

 masses of tubules loosely held together. They not only 



* The application of the term "cardiac" to this organ is a 

 reductio ad absurdwn of the plan of naming parts of Invertebrate 

 animals after the vaguely or fancifully corresponding parts of 

 human anatomy.. The " cardiac " part of the human stomach is so 

 called because it is the part nearer the heart ; In the crayfish the 

 " cardiac " region is away from the heart. 



