40 General Part. 



fluid, which is for the most part to be regarded as exuded blood 

 plasma. Such are the synovial cavities of Vertebrata. 



12. Rudimentary Organs. 



Besides the great majority of organs which evidently perform 

 definite functions, structures are occasionally found which are of no 

 importance to the animal, the so-called rudimentary organs. 



The hind leg of the Greenland Whale may be cited as an example 

 of such an organ. It consists of a femur and a tibia, both of which are 

 concealed beneath the skin, and have entirely lost their function. 

 The " wolf tooth " of the Horse affords another example of a 

 rudimentary organ ; that in the lower jaw especially, is an instance of 

 extensive reduction, for, though it is formed in the customary way, it 

 is very seldom cut. The eyes of Myxine (the hag-fish), Proteus, and 

 many other blind animals; the wings of Apteryx (the kiwi) and 

 other Struthious Birds ; the minute wings of many female Butterflies 

 are examples of rudimentary organs. 



It may seem strange that these rudimentary and functionless 

 organs are so generally present, but further consideration renders 

 their existence less incomprehensible. Rudimentary organs which 

 are now useless, were, in ancestral forms [see Section V.), functional 

 and useful parts. The adaptation of the animal to a new and peculiar 

 environment rendered them useless, and during the development of the 

 existing form, they became reduced and functionless. It must, for 

 example, be admitted that the Whale is descended from a Mammal, 

 which, like most other Mammals, was provided with well-developed 

 hind limbs, but these were gradually reduced by the adaptation of 

 the aninial to an aquatic life, while the tail came to function as the 

 essential locomotor apparatus. A similar explanation can be given 

 for the other cases cited above. 



This explanation does not, liowever, hold for every rudimentary structure. 

 Such parts as the rudimentary mammary glands of male Mammals, the rudi- 

 mentary oviducts of male Amphibia, the rudiment of a copulatory organ in many 

 female animals, etc., are to be accounted for in another way. Those parts which 

 are always present in one sex in a well- developed and functional condition have 

 probably been transmitted by this sex to the other. The mammary glands, for 

 example, were probably, first of all, present only in the female, and have been 

 inherited by the male ; the copulatory organs, conversely, usually present at first 

 only in the male, have then been transmitted to the female in an incipient state. 



III. Fundamental Form and External 

 Configuration of the Body. 



In a small number of animals, Bchinoderma and Coelentera, 

 the body is so constructed that it may be divided into a number 

 of nearly congruent radially-arranged segments, antimeres (rays) : 



