78 ORGANIZATION AND DEVELOPMENT OF ANIMALS IN GENERAL. 



Mn 



coloured and variously shaped shells of these animals. Integumen- 

 tary glands and aggregations of glands may also acquire a relation 

 to the acquisition of food (spinning glands 

 of Spiders). Finally, mucous glands are 

 i*v very widely present in the skin of animals 

 which live in damp localities (Amphibia, 

 Snails) and in water (Fishes, Annelids, 

 Medusae). 



C ^ ORGANS OF ANIMAL LIFE. 



Of the so-called animal functions, that 

 of locomotion is the most conspicuous. 

 Animals perform movements for the 

 purpose of procuring food and escaping 

 from their enemies. The muscles used 

 for locomotion are, as a rule, and especially 

 in the simpler forms, intimately united 

 with the skin, and give rise to a muscular 

 body wall (Worms), the alternate shorten- 

 ing and elongation of which brings about 

 a movement of the body. The muscles 

 may also be especially concentrated in 

 parts of the body wall, e.g., in the subum- 

 brellar surface of Medusae beneath the 

 supporting gelatinous tissue, or in the 

 ventral surface of the body giving rise 



to a foot-like organ (Molluscs), or they may be broken up into 

 a series of successive and similar segments (Annelids, Arthropods, 

 Vertebrates). The latter arrangement prepares the way for the 

 rapid and more complete form of movement found in animals in 

 which the hard parts also, whether exoskeletal (Arthropods) or 

 endoskeletal (Vertebrata), have become divided into a seiies of 

 longitudinally arranged segments or rings, which offer a firm attach- 

 ment to and are moved by the segments of the muscular system. 

 By this arrangement more powerful muscular actions are rendered 

 possible. 



Thus it becomes indispensable that hard parts should be developed 

 to act as a skeletal support for the soft parts, and also to protect them. 

 The skeletal structures may be external, in which case they have the 

 form either of external shell?, tubes or successive rings, and are 



Ail 



FIG. 73. Alimentary canal with 

 its accessory glands of a beetle 

 (Carabus) (after Leon Duf our). 

 Oe, oesophagus ; Jn, crop ; Pv, 

 proventriculus ; Chd, chylific 

 ventricle ; My, Malpighian tu- 

 bules ; S, rectum ; Ad, anal 

 glands with bladder. 



