PRINCIPLES OF ANIMAL BIOLOGY 55 



86. Body bilaterally symmetrical (see Chiton, Anodonta, and the 

 cuttle-fish Sepia), though often apparently asymmetrical (the common 

 snail Polygyra, Natica, or any other snail). 



Sc. Locomotion usually by a fleshy, muscular foot. In Modiola, 

 Lampsilis, and the clams generally, the foot is wedge-shaped. In Poly- 

 gyra and other snails, it is flat and used for creeping along surfaces. In 

 Jjoligo, Sepia, and N utilus, the foot is composed in part of a series of 

 arm-like projections. 



Sd. Body usually protected by a calcareous shell which may consist 

 of two valves (Lampsilis, Pecten, Pinna), of a spirally wound tube (Helix, 

 Polygyra, Nautilus), or be concealed by the fleshy parts (Loligo, Sepia), 

 or wanting (Octopus). 



Se. Possess a mantle, a thin membranous sheet that secretes the 

 shell. This may be a single piece (Polygyra, Loligo), in two flaps lining 

 the two valves of the shell (Lampsilis, Modiola), or wanting (the slug 

 Limax). 



9. Phylum ARTHROPODA (Gr. arthros = joint + pous = foot). The 

 Arthropoda include hundreds of thousands of species, probably a greater 

 number than any other phylum. 



A. Characteristics. 



9a. Segmented. Examine any insect; a crayfish; a centipede; a 

 spider. 



96. Paired jointed appendages (legs, antennae, mouth parts, etc.). 

 How many pairs of legs in an insect? In a crayfish? In a centipede? 

 In a spider? 



9c. An exo-skeleton of chitin covering the body. Observe in all the 

 forms mentioned in the preceding paragraphs. 



B. Special Considerations. 



9rf. Although each arthropod has a definite number of segments in its 

 body, these segments are often fused in characteristic ways so that the 

 number is not easy to determine. The number of appendages, or the 

 embryonic development, is relied on in such cases to establish the 

 correct number. 



In insects, a number of segments are fused to form a head, others are 

 fused to form a thorax, while the segments of the abdomen remain more 

 or less movable upon one another. Make out these regions in Polistes 

 (wasp) and Dissosteira (grasshopper), or other insects. 



In the crayfish, the segments of both head and thorax are fused into 

 one immovable group called the cephalo-thorax, while those of the abdo- 

 men are movable. Make out these regions in Cambarus and Palinurus, 

 or other crayfishes. 



In spiders, the cephalo-thorax is one group of fused segments and the 



