200 AN INTRODUCTION TO ZOOLOGY 



represents the condition from which the other types developed. 

 The uniramous walking legs, for example, pass through a 

 biramous stage during their embryological development. Again, 

 the biramous embryonic maxillipedes are converted into the 

 foliaceous type by the expansion of their basal segments (166). 



General Internal Anatomy. The body of the crayfish 

 contains all of the important systems of organs characteristic of 

 the higher animals. The ccelom is not a large cavity, as in the earth- 

 worm, but is restricted to the cavities of the reproductive organs. 

 Certain of the organs are metamerically arranged, e.g. the nerv- 

 ous system; others like the excretory organs, are concentrated 

 into a small space. The systems of organs and their functions 

 will be presented in the following order: (i) digestive, (2) vas- 

 cular, (3) respiratory, (4) excretory, (5) nervous, (6) sense organs, 

 (7) muscular, and (8) reproductive. 



Digestive System. The alimentary canal of Cambarus con- 

 sists of the following parts (Fig. 101) : 



(1) The mouth (4) opens on the ventral surface between the 

 jaws (3). 



(2) The oesophagus (20) is a short tube leading from the mouth 

 to the stomach. 



(3) The stomach is a large cavity divided by a constriction 

 into an anterior cardiac chamber (21) and a smaller posterior 

 pyloric chamber (22). In the stomach are a number of chitinous 

 ossicles of use in chewing the food, and collectively known as the 

 gastric mill. The most important of these are (a) a median car- 

 diac ossicle, (b) a median urocardiac ossicle, (c) two lateral ptero- 

 cardiac ossicles, (d) a pair of lateral zygocardiac ossicles, (e) a 

 pyloric ossicle, and (/) a prepyloric ossicle. The ossicles are able 

 to move one upon another, and, being connected with powerful 

 muscles, are effective in grinding up the food. On either side of 

 the pyloric chamber enters a duct of the digestive glands, and 

 above is the opening of the small caecum. 



At certain times two calcareous bodies, known as gastroliths, 

 are present in the lateral walls of the cardiac chamber of the 



