4 88 CHARACTERS OF INVERTEBRATE ANIMALS 



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constitutes the essential part of all organisms, in a comparatively 

 pure form, and not obscured to the same extent as in higher 

 animals by the products of its own activity. It has become a 

 tradition to take as a first type one of the simplest members of 

 the group, i.e. the Proteus Animalcule or Amoeba, which is 

 commonly found creeping on the mud of ponds. 



Reference has frequently been made to the microscopic 

 bodies known as white or colourless corpuscles which abound 

 in the blood and lymph of higher animals (see p. 41). These 

 crawl through the body in all directions and perform various 

 functions of no mean importance, one being that of destroying 

 disease germs which have entered the organism from the exterior. 

 These corpuscles may almost be said to lead an existence inde- 

 pendent of the rest of the body, and indeed it is possible to keep 

 them alive outside the organism to which they belong for some 

 time, especially in the case of cold-blooded creatures such as the 

 Frog. An Amoeba resembles in many essential respects one of 

 these corpuscles, so much so that when it cannot be obtained 

 for study in the laboratory a white corpuscle is often taken as 

 the best substitute. The body (fig. 301) consists of a particle of 

 semi-fluid protoplasm possessing the power of active locomotion, 

 employed in the search for food, which consists of microscopic 

 plants and other solid bodies of organic nature. The complex 

 and solid nature of the food, or part of it, is, as will be elsewhere 

 shown, a characteristic of average animals as compared with 

 average plants, and the powers of locomotion with which most 

 animals are endowed has an obvious relation to this. In such 

 a fixed animal as a sponge there is, as we have seen, a special 

 arrangement by which food is brought to the body, compen- 

 sating for the absence of locomotor powers. An ordinary green 

 plant, feeding as it does upon gaseous and liquid food extracted 

 from air and soil, has no need for powers of locomotion, and its 

 branching form gives a very large surface through which the 

 simple food can diffuse. An active animal, on the other hand, 

 such as Amceba, has a compact body which is clearly more con- 

 venient for locomotor purposes and less exposed than a branching 

 form would be to the attacks of enemies. When tree-like organ- 

 isms, such as zoophytes, are of animal nature, they usually represent 

 fixed colonies to which food is brought by currents. 



The body of a living Amceba is seen to be constantly changing 



