Introductio7i to Animal Morphology. i i 



into systems is morphological, into apparatuses, phy- 

 siological. 



The simplest animals are plastides or homoplastic 

 aggregates. In higher forms greater differentiations 

 occur, and when heteroplasts are grouped into organ- 

 systems, these are generally symmetrically arranged, 

 similar heteroplasts being placed side by side around 

 a horizontal axis. Thus a star-fish consists of segments 

 or groups of organ-systems, each built upon the same 

 plan, placed radially around a centre. Such segments 

 may be called antimeres [Haecket). They vary in 

 number : each segment of a bilaterally symmetrical 

 animal (Vertebrate or Arthropod) has two ; some 

 Radiolaria, three ; Medusae, four ; many Echinoderms, 

 five ; Zoantharia, six ; Luidia, seven ; Ctenophora, 

 eight, &c. The increase of a function may be pro- 

 vided for by a multiplication of organs, or a more 

 complicated development in those already existing. 

 Of these methods the former seems the simpler, hence 

 the next step in complexity is the formation of a chain 

 of similar groups in succession. To each element in 

 this chain is given the name metamere. These seg- 

 ments may be similar, as in the case of the zonites of 

 a worm, or else some may be specialized. The whole 

 chain constitutes a persona* in which the metameres 

 are usually successional (catenated), or else lateral 

 (fruticose). The highest degree of aggregation in the 

 animal kingdom is the colony, in which a number of 

 personae are united on a common stem as among the 

 compound Hydrozoa. 



* The name persona may be given to one of the component elements of 

 a colony, whether it be built up f metameres or no, although strictly it 

 should be limited I o such as are so composed. 



