GENERAL ORGANOLOGT. 101 



organs which carry on digestion. Hence we may group all organs 

 which in the body have equivalent or similar functions, and speak 

 of systems of organs. In all we recognize nine such systems: (1) 

 skeletal, (2) digestive, (3) respiratory, (4) circulatory, (5) excre- 

 tory, (6) genital, (7) muscular, (8) nervous, and (9) sensory 

 systems. Not all are necessarily present; a skeleton, for instance, 

 is frequently lacking. The most different functions which in man 

 are divided among different complicated and specialized systems 

 may be performed in a lower animal by one and the same 

 apparatus. Yet according to the fundamental functions the fol- 

 lowing groups of organs may be recognized : I. Organs of assimila- 

 tion (2-5); II. Organs of reproduction (6); III. Organs of motion 

 (7) ; IV. Organs of perception (8 and 9). 



Vegetative and Animal Organs. The organs of assimilation and of repro- 

 duction (I and II) are grouped together as vegetative, the others (III and 

 IV) as animal organs. The older zoologists used to say that assimilation 

 and reproduction are functions which are common to animals and plants ; 

 that, on the contrary, sensation and motion are lacking in plants, and are 

 exclusively characteristic of animals. The atom of truth in the funda- 

 mental idea needs reconsideration in the light of our present knowledge. 

 We have seen that the protoplasm of plants and animals has not only the 

 power of assimilation and reproduction, but also power of motion and of 

 irritability. The latter characteristics consequently cannot be entirely 

 lacking in all the plants, for they are found in the most important. In 

 fact many plants, as the mimosas, the compass-plants, insectivorous plants 

 show great irritability ; many low plants, the reproductive states of algae, 

 move quite as actively as, or even more actively than, many of the lower 

 animals. On the other hand, there are many animals which in the mature 

 condition are fixed in position like plants. Many Protozoa and worms, 

 most of the zoophytes, some echinoderms like the Crinoids, even many 

 Crustacea, the cirripedes (barnacles), can change their location only during 

 the earlier stages of development, in later life being limited to movements 

 of single parts of the body, the arms, tentacles, etc. In the sponges 

 motions are so insignificant that they cannot be seen at all by the naked 

 eye, and scarcely even with the aid of the microscope. 



Nevertheless the two terms, animal and vegetative, must be retained. 

 For although motion and sensation occur in the vegetable kingdom, still 

 they reach no high development ; indeed we may say they become more 

 and more inconspicuous the higher the plants ; in the animal kingdom, on 

 the contrary, they are unfolded in extraordinary perfection and lie at the 

 basis of its most characteristic features. 



