COMPARATIVE ANATOMY. 



115 



COMPARATIVE ANATOMY. Ill 



NS OP THE ANIMAL KINGDOM TABLB Or SUB- 

 DIVISIONS OB CLAB8BB PROTOZOA. 



TMK main divisions into which the animal kingdom i divided 

 iveii in the last lesson. We have now to suh-dividf 

 these branches into classes, a class being tho next lower deno- 

 mination in our arrangement. The order in which we now 

 enumerate the classes will be an ascending one. We shall 

 proceed from the lowest animals to the highest. This order is 

 fviiK-ntly tho more scientific, because, other things being omitted 

 from tho consideration, it is always easier to proceed from tho 

 more simple to the more complex. Thus, in chemistry it is 

 usual first to treat of the 

 elements, or tho most sim- 

 ple forms of inert matter, 

 and then to proceed to com- 

 pounds, made by the unions 

 of these elements. By this 

 method of procedure it is 

 true that wo deprive the 

 student of anatomy, who 

 has just commenced the 

 study, of a valuable aid. 

 In studying the higher 

 animals, a knowledge of 

 his own body, however su- 

 perficial, the consciousness 

 of what he feels, however 

 faulty the inductions there- 

 from, and the constant pre- 

 sence of a similar living 

 subject on whom to expe- 

 riment, all furnish illustra- 

 tions to the tyro of struc- 

 ture in relation to life. 

 When, however, we begin 

 by a description of animals 

 so far removed from him- 

 self as the Protozoa, there 

 is nothing which he learns 

 about them which fits in 

 with his previous concep- 

 tions. The animals them- 

 selves con inspire him with 

 little interest as compared 

 with those higher beings 

 with whom we have not 

 only an essential community 

 of structure, but whose very 

 instincts and sympathies 

 we share. The student, 

 however, should remember 

 that his conceptions of the 

 phenomena of life, and his 

 interest in animals, will 

 dawn upon him with greater 

 clearness and a more genial 



warmth, as he proceeds and I> AM<EB * (*"EB PBITCHAED). II. SHELL or POLTCTSTTHA (MUCH MAOMI- 

 arrives at the study of the " ED) ' - SECTIONAL DIAOEAK SHOWIHO THE CIECULATIOH w A SFOKGE 



higher forms by \aving l ^S^ J^ " Vo ~ tt <* D) ' ^ Hocn " 



first mastered the succes- 

 sive steps by which the great problem of life has been worked 

 out. One question must be answered before we can proceed 

 along the road which we have decided to be the best and safest. 

 When may an animal be said to be higher or lower than another 

 in the scale of tho animal creation ? The self-complacent 

 assumption that man is the highest animal, and that an animal 

 is higher or lower in proportion as he is more or less like to 

 him, will serve us but little ; because, as we have seen, animals 

 cannot be ranged in a single series, and there is no link of 

 connection between some of the branches we have given and 

 that to which we belong. One principle alone should guide us 

 in our arrangement, and that is illustrated by the following 

 axiom : An animal may be said to be higher than another if 

 its structure is more complex. If an animal performs many or 

 all of the functions of life by or through one structure, it is low 

 in the scale ; but if certain parts are separated and assigned 



34-x.K. 



iv 



to the performance of definite rital act*, which acts an there- 

 fore done more efficiently, then the animal oocapiM a hi* her 

 station. Just as political eooaomiiti will tell n that nitiom 

 are more or less civilised in proportion as the principle of th 

 sub-division of labour is followed out in their commnnitie*, to 

 the comparative anatomist will esteem that animal the hifhwt 

 whose body is made up of the greatest number of difcrtnt 

 organs. This idea baa given origin to a *4**htMal term, which, 

 though barbarous English, is useful for ""tvitfnoai. When 

 the reader meets with the expression that an animal mintfttti 

 greater differentiation of partt than *"thw t be will henceforth 

 know not only what is meant, but also the bearing* of the 

 statement. Differentiation, however, most be <*'*igH^ 



from multiplication of 

 parts. The poaweaion by 

 an animal of many organ* 

 all alike in function and 

 structure, is a sign rather 

 of a low than of a high ela- 

 tion. A worm's body is 

 composed of very many 

 rings, while the bee has 

 only thirteen at most; but, 

 with the exception of the 

 head and tail segment of 

 the worm, they are all alike, 

 while every ring of the bee 

 is dissimilar. This latter 

 animal, therefore, is much 

 higher, because it mani- 

 fests a greater differentia- 

 tion of organs. 



The tendency of an ani- 

 mal to produce a repeti- 

 tion of the same structure, 

 whether the structures so 

 formed be set forth in a 

 radical manner or in a lon- 

 gitudinal series, is called a 

 " vegetative repetition of 

 parts." 



The most prominent fea- 

 ture in the vegetable f o 

 which clothe the! 

 and which therefore give 

 to us our ideas of vegetable 

 life, is their unlimited 

 growth, resulting in the 

 almost infinite production 

 of like parts. The forest tree 

 has ten thousand leaver 

 all built upon a framework 

 of netted veins of the same 

 pattern. When the chest- 

 nut puts forth its " milky 

 cones," it does so in rich 

 profusion, and each of these 

 bean many flowers, all 

 alike. Thus, the phrase 

 vegetative repetition is de- 

 rived from the most promi- 

 nent feature presented to 



and impressed upon the mind as associated with vegetable life. 

 Combining this idea of vegetable growth with the knowledge 

 that the vegetable kingdom has never developed any organic 

 form at all comparable for the exhibition of the higher functions 

 of life to those which wo find in the animal kingdom, we assume 

 that this vegetative repetition of parts is a sign of a low grade 

 of structure : so doubtless it is, but it is not a sign of ths Imnut 

 grade. In tracing upward tho different forms assumed by living 

 animals, the idea strikes toe mind that Nature's first effort was to 

 form a simple individual like a cell. Tho next step seems to be 

 an indefinite multiplication of similar celliform bodies in such 

 connection with one another as to form an aggregate individual, 

 and then these parts become more specialised and consequently 

 more interdependent, and at the same time more definite and 

 fewer in number. 



In the following table we have, as far as it was possible, 



