134 AN INTRODUCTION TO ZOOLOGY 



primitive kind of indirect division, but all the changes take place 

 entirely within the persistent nuclear membrane, and no extra 

 nuclear spindle or centrosomes are formed. 



If we compare briefly Amceba and Paramcecium we see 

 that the latter is a more complex animal than the former. In 

 Amceba we have a comparatively simple mass of protoplasm, almost 

 an ideal primitive cell, that is capable of exhibiting all the vital 

 phenomena, almost the only specialised part being the contractile 

 vacuole. Paramcecium, on the other hand, has a definite protective 

 and supporting cuticle, a locomotor apparatus in the form of cilia, 

 excretory pulsating vacuoles, contractile myoneme fibrillge, a 

 cytostome, cytopharynx, trichocysts, and so on. All of these are 

 definite structures which can perform special functions that in 

 Amceba are carried out by the general protoplasm. Here we have 

 an example of a very important principle underlying animal organisa- 

 tion, namely, the division of physiological labour, accompanied by 

 the production of special parts to subserve definite functions, or, as 

 we say, by a differentiation of structure. The higher we pass in the 

 scale of animal life the more complete is the division and the more 

 elaborate the specialisation. The frog affords an illustration of this, 

 for not merely are certain parts separated off for set purposes, but 

 the one function itself may be subdivided. Thus the function of 

 digestion carried out in Amceba and Paramcecium by the food 

 vacuoles, is delegated to the alimentary system in Rana, and this 

 in itself is composed of a number of separate organs, each with a 

 limited part to play. One result of this is that the various bodily 

 activities can be carried out with the maximum efficiency. On the 

 other hand, however, when a tissue has become so highly specialised, 

 it can do one thing only and nothing else, so that the maintenance of 

 life depends on the activity of large separate parts. If any one 

 of these ceases to function, life stops, and so we find a number of 

 " vital organs," the elimination of any one of which by accident 

 or disease brings about the death of the whole animal. A com- 

 parison is often drawn between this separation of functions in the 

 animal body and the division of an industrial community into 

 different trades and occupations of varying utility to the remainder. 

 The analogy, although not perfect, is fairly close, and here again the 

 complete cessation of work on the part of certain classes of operatives 

 would bring the whole industry to a standstill. This would only 

 be temporary, however, since men are not so highly specialised that 

 they are incapable of doing any other sort of work, and a fresh set 

 of workmen could soon be trained. In the community, as in the 

 animal, increased efficiency is purchased at the price of increased 

 vulnerability. 



