I08 OUTLINES OF NATURAL HISTORY. 



covering or shell. This sub-kingdom includes the classes 

 of the Folyzoa, Ttmicata^ Brachiopoda, Lamellibranchiata, 

 Gasteropoda, Pteropoda, and Cephalopoda, which agree 

 in the possession of the following characters : — The ani- 

 mal is soft-bodied, and does not exhibit either distinct 

 segmentation, or any star-like arrangement of its parts. 

 Usually, but not universally, the soft body is protected 

 by a hard outer covering or shell. The body-cavity does 

 not communicate with the outer world through the 

 mouth, and there is always a well-developed digestive 

 system which in no case communicates directly with the 

 body-cavity. The nervous system consists of a single 

 principal mass or of scattered masses not arranged in a 

 chain. There is usually a distinct heart, and generally 

 distinct breathing-organs, but both may be absent. Ex- 

 amples of this sub-kingdom are the Sea-mats, Sea-squirts, 

 Lamp-shells, Bivalve Molluscs (Oysters, Mussels, and the 

 like). Univalve Molluscs (Snails, '\^^lelks, and the like), 

 Cuttle-fishes, &c. 



SUB-KINGDOM VI. VERTEBEATA. 



This sub-kingdom derives its name from the fact that 

 the animals comprised in it have a more or less perfectly 

 developed backbone (Latin, vertebra, a bone or joint of 

 the spine or backbone). The sub-kingdom includes the 

 classes of the Pisces (Fishes), Amphibia, Rejytilia, Aves 

 (Birds), and Mammalia (Quadrupeds), all of which agree 

 with one another in the following characters : — The body 

 is composed of a nimiber of definite segments placed one 

 behind the other in a longitudinal series. The body-cavity 

 never communicates with the outer world through the 

 mouth, and there is always a well-developed alimentary 

 canal which never opens into the body-cavity. A distinct 

 blood-system and distinct breathing-organs are invariably 

 present. The nervous system is placed in a cavity dis- 

 tinct from the general body-cavity, and is situated along 

 the back of the animal. With one exception, the ner- 



