42 ESSENTIALS OF BIOLOGY 



the sedentary or sessile habit the structure becomes modified in 

 so far as organs of locomotion are not at all, or are feebly 

 developed. 



no. Shelled Types. Many kinds of animals have bodies 

 that are naked, in the sense that they are not covered with hair, 

 scales, or other integumental, protective structures. Thus there 

 are the naked Sea-anemones and Sponges, the Marine Nudibranch 

 Molluscs, the Garden Slugs, etc. Many other forms are wholly 

 or partially enclosed in a hard stony shell — the Oyster and other 

 lamellibranch Molluscs, the common Barnacles, Whelks, etc., the 

 Sea-urchins are examples. There are animals with bodies 

 thickly covered with fur or feathers (as with most Mammals and 

 Birds), or with scales (Fishes and Snakes) and so on. Such forms 

 are intermediate, in a way, between the naked and shelled types. 

 In general the shells, scales, fur, feathers, etc., are to be regarded 

 as structures secreted by the skin, for protective purposes and 

 they are permanent (as in the Molluscs) or may be cast off and 

 renewed (as in the cases of the Crustaceans and many other 

 animals). 



lip. The Parasitic Types. Many groups of organisms 

 have so evolved that they have lost the power (either wholly, or 

 periodically, at some phase in their life-history) of independent 

 life in open nature. They are resident as parasites, in or on the 

 bodies of other animals. Examples of such groups are : many 

 kinds of Bacteria and Moulds, Cestodes (or Tapeworms), Trema- 

 todes (Liver-flukes), Nematodes (Threadworms), etc. In these 

 parasitic forms (and particularly those that have thoroughly 

 evolved the parasitic habit) there are profound, and often bizarre, 

 modifications of structure and habit. In all of them some notion 

 of the free-living ancestral forms from which they have evolved 

 can be made out from a study of the embryologies. 



These types of animal bodies and structures are intended only 

 to give some general idea of the forms of life in the most general 

 sense. The study of phylogeny (or blood-relationships con- 

 sequent upon the evolutionary process) demands a knowledge 

 of rational classifications and the study of individual development 

 means the investigation of embryonic structures. But for the 

 study of organic functioning in general the investigation of 

 structure (though it is always made) is not necessarily informative. 



