HIGH SCHOOL ZOOLOGY. 



191 



haemal aspects, as compared with the Vertebrates (Fig. 119) ; 

 nevertheless, it is the neural aspect which is iirst developed in 

 both sub-kingdoms. No endoskeleton affords attachment to the 

 muscles of the body or protects the delicate organs, but this is 

 functionally replaced by an exoskeleton of chitin, a hard sub- 

 stance of peculiar chemical composition, secreted by the skin, 

 and sometimes rendered harder by the admixture of calcareous 

 salts. 



Fig. 119.— Diagram of transections through the abdominal regions of a catfish and 

 a crayfish, to show the relative position of nervous system, N, and intestine, I. 

 D, dorsal ; V, ventral surface ; E, endoskeleton ; B, aorta ; K, kidney. 



3. The last peculiarity is especially met with in the Crustacea, 

 one of the four Arthropod classes, and that to which the crayfish 

 belongs. The other classes (Insecta, Araclinida, Msrriapoda) 

 embrace chiefly air-breathing Arthropods, whilst almost all 

 Crustacea are aquatic, so that there is, on the whole, a marked 

 difference between the respiratory organs of the Crustacea and 

 those of the other classes. 



Several species of crayfish or crawfish (old English crevish, 

 Fr. ecrevisse, Ger. Krel)s) occur in Ontario ; one of the com- 

 monest near Toronto is Comharus robustus, Girard, (Fig. 120), 



