CHARACTERS OF THE SUB-KINGDOMS. 17 



corpuscles; the embryo is provided with amnion and 

 allantois ; gills are never present during any period of 

 life ; the skull is joined to the vertebral column by a 

 single condyle (Gr. condulos, a knuckle) on the occipital 

 bone; each half of the lower jaw consists of several pieces; 

 the lower jaw is joined to the skull by a bone called the 

 quadrate (Lat. quadra, a square) bone. These two classes 

 form the division Sauropsida (Gr. sauros, a lizard). 



13. The Mammalia form a distinct section. They agree 

 with the Sauropsida in being destitute of gills, and in 

 possessing an amnion and allantois; but differ from them 

 in having circular red blood-corpuscles, without any 

 nucleus, in possessing glands for the secretion of milk 

 (mammary glands), and in being generally covered with 

 hair. There are two condyles on the occipital bone, each 

 half of the lower jaw consists of a single piece, and is 

 united directly with the skull, and not to a quadrate 

 bone. 



SUB-KINGDOM II. Annulosa. 



14. This sub-kingdom, which is also sometimes termed 

 Articulata, comprises those animals that consist of a num- 

 ber of rings or segments, arranged behind one another 

 in a straight line. There is no skeleton properly so called, 

 but the hardness of the external covering of most of them 

 compensates for the absence of the internal framework 

 found in the Vertebrata. This hardness is sometimes 

 due to lime, which, in crabs, lobsters, &c. (Crustacea) 

 forms a hard external crust. 



Sometimes the cuticle is hardened by a chemical sub- 

 stance termed chitine (Gr. chiton, a tunic), allied to horn, 

 the skin, in such cases, somewhat resembling a tanned 

 hide. This is the nature of the covering in the insects, 

 centipedes, &c. To these hardened rings the muscles are 

 attached. 



There is always a nervous system which is situated 

 below the alimentary canal. It generally consists of a pair 

 15 E. B 



