8 THE HUMAN BODY. 



group as fishes, reptiles, birds, and beasts* : sea ane- 

 mones, clams, and insects are invertebrate animals, and 

 built on quite different plans ; sections made through 

 any of them from the head to the opposite end, would 

 show nothing like those two main cavities with a backbone 

 between them which exist in our own bodies. 



Contents of the two chief cavities of the body. Exam- 

 ination of Fig. 1 shows that the ventral cavity is entirely 

 closed itself, though some things which lie in it are hollow 

 and communicate with the exterior. On the head we 

 find the nose, i, and the mouth, o, opening on the ven- 

 tral side ; that is on that surface of the body next which 

 the haemal cavity lies. The nose chamber joins the mouth 

 chamber at the throat, and from the throat two tubes 

 run down through the neck and enter the ventral cavity. 

 One of these tubes, placed on the ventral side of the other, 

 is the windpipe, and leads to the lungs, I; the other is the 

 gullet, and leads to the stomach, f. From the stomach, 

 another tube, the intestine, leads to the outside again at 

 the lower or posterior f end of the trunk. Mouth, throat, 



What fact in man's anatomy makes him a vertebrate animal? 

 Name some other vertebrate animals. Name some invertebrates. 

 How would sections made through invertebrates differ essentially 

 from similar sections made through a man? 



Is the ventral cavity open? Do any smaller cavities in it open on 

 the exterior of the body? What openings do we rind on the head? 

 What is the windpipe? To where does it lead? What is the intes- 

 tine? What parts constitute the alimentary canal? Does this canal 

 lie entirely within the haemal cavity? 



* The main groups m which animals arc arranged are 1. Vertebrata, or backboned 

 animals. 2. Mollusca, including snails, slugs, clams, oysters. &c. 3. Arthropoda, 

 including flies, moths, beetles, centipedes, lobsters, spiders, &c. 4. Vermes, includ- 

 ing worms of various kinds. 5. Echinodermata (hedgehog-skinned animals), in- 

 cluding sea urchins, star fishes, &c. 6. Cailenterala, the sea-anemones and their 

 allies. 7. The Protozoa ; all microscopic and very simple in structure. 



t In anatomy the head end of an animal is spoken of as anterior, and the oppo- 

 site end as posterior, no matter what may be the natural standing position of the 

 creature, 



