25 



by the term organ every part of the body which as a unit subordi- 

 nate to the higher unit of the organism presents a definite form and 

 structure, and performs a corresponding function ; that is to say, an 

 organ is one of those numerous instruments on the combined work- 

 ing of which the life of the individual depends. 



There are certainly among the simpler animals many instances in 

 which the term individual in its usiial sense cannot be rightly 

 applied. In such cases we have to do with structures which from 

 their development must be termed individuals, and represent indi- 

 viduals, accordingly, in a morphological sense. A great many of them 

 are, however, fused to a common stock, forming what is known as a 

 colony, and are related physiologically to this, as organs are to an 

 organism. They are accordingly incomplete or morphological indivi- 

 duals, which are usually incapable of leading a separate existence ; 

 and, when they differ from each other in form and function, dividing 

 amongst themselves the labours, the performance of which is neces- 

 sary for the maintenance of the whole colony, they always perish 

 if separated from it. 



Such polymorphous* stocks of animals present the properties of 

 individuals although they are morphologically aggregations of indi- 

 viduals which behave physiologically as organs (fig. 5). On the other 

 hand, groups of organs can acquire individual independence. 



In the animal body organs do not always remain single, but the 

 same organ may be often repeated. The manner of the repetition is 

 dependent on the kind of symmetry, which may be radiate or bilateral. 

 In animals with radiate symmetry, the Racliata, it is possible to 

 connect two opposite points of the body by an axis, which may be 

 called the chief axis, and to divide the body by sections passing 

 through this axis into a number of equivalent and symmetrical parts 

 known as antimeres. The organs which are not repeated are situated 

 in the chief axis of the body, while the other organs, which are 

 uniformly repeated in each antimere, are situated peripherally. Eacli 

 antimere contains, therefore, a definite group of organs and represents 

 a secondaiy unit, which, together with its fellows and the central 

 organs, constitutes the primary unit, i.e., the perfect animal. 



In a radiate animal a number of lines can be drawn at right angles 

 to the chief axis, corresponding in number to the antimeres, and 

 each passing along the middle of an antimere; such lines are known 

 as radial. Similarly, a corresponding number of inter-radial lines 



* Vide E. Leuckart, " Ueber den Polymorphismus der Individuen and die 

 Erscheinung der Arbeitstheilung in der Natur." Giessen, 1851. 



