242 HOW DO WE CLASSIFY ANIMALS? 



the figure of the squid shows, the mouth is surrounded with a 

 circle of tentacles. The shell is internal or lacking. 



To this group of animals belong also the octopus, or devilfish ; 

 the paper nautilus ; and the pearly nautilus. 



Practical Exercise 11. From a study of the diagrams and of the text, make 

 up a good definition of a mollusk. What mollusks are common in your 

 environment ? 



Mollusca. These animals are soft-bodied animals, often pro- 

 vided with a shell, which is secreted by a part of the body called 

 the mantle. They usually have a single muscular foot on the 

 ventral side. Over 60,000 species are known. There are five 

 classes of mollusks, but only three classes are widely known. 



Self-Testing Exercise 



A mollusk is a (1) animal. It usually has a (2) 



which is (3) by the mantle. This (4) is either 



(5) as in the oyster, or ........ (6) as the snail. Most 



mollusks, living in the water, take in waiter through (7) tubes 



called (8). Common examples of mollusks are (9), 



(10), and (11). 



PROBLEM V. WHAT ARE THE CHARACTERISTICS OF FISHES? 



The animals we have studied thus far agree in having any skeleton 

 they may possess on the outside of the body. They are called 

 invertebrates. In higher animals, of which the fish is an example, 

 the skeleton is inside the body. They are called vertebrates. While 

 the exoskeleton of invertebrates is dead material secreted by the 

 body cells the endoskeleton of vertebrates is made up of cartilage 

 and bone, living material, capable of growth and repair. The 

 skeleton of all Vertebrates has two main divisions, the axial skeleton, 

 consisting of the skull and vertebral column and the appendicular 

 skeleton, consisting of two pairs of limbs together with the girdles, 

 the bones, by which the limbs are attached to the vertebral column. 

 The vertebral column and skull protect the delicate spinal cord 

 and brain. The limbs support the body and aid locomotion. 

 Vertebrates are segmented but these segments have been highly 

 modified to form the various organs. Vertebrate animals deserve 

 more of our attention than other forms of life because man himself is 



