STRUCTURE AND CLASSIFICATION OF THE ARTHROPODA. 531 



case of a tendency of the organic body to repetition of struc- 

 tural units or parts wliicli finds one expression in bilateral 

 symmetry. In certain worms (the Cestoidea and some 

 Planariaus) metameric segmentation is accompanied by the 

 separation of the completed metameres one by one from the 

 older (anterior) extremity of the chain (strobilation), but it 

 by no means follows that metameric segmentation has a 

 necessary origin in such completion and separation of the 

 " meres." On the contrary, metamerism seems to arise from 

 a property of organisms which is sometimes more (eumero- 

 genesis) and sometimes less (dysmerogenesis) fully exhibited, 

 and in some groups not exhibited at all. The most complete 

 and, at the same time, simplest instances of metameric seg- 

 mentation are to be seen in the larger Ch^topods, where some 

 hundreds of segments succeed one another — each practically 

 indistinguishable in structure from the segment in front or 

 from that behind ; muscles, right and left appendage or 

 parapodium, colour pattern of the skin, gut, blood-vessels, 

 coelom, nephridia, nerve-ganglion, and nerves are precisely 

 alike in neighbouring segments. The segment which is least 

 like the others is the first, for that carries the mouth and a 

 lobe projecting beyond it — the prostomium. If (as sometimes 

 happens) any of the hinder segments completes itself by 

 developiug a prostomium, the chain breaks at that point, and 

 the segment which has developed a prostomium becomes the 

 first or head-bearing segment of a new individual. Compare 

 such an instance of metameric segmentation with that pre- 

 sented by one of the higher Arthropods — e. g. the crayfish. 

 Here the somites are not so clearly marked in the tegumentary 

 structures; nevertheless, by examining the indications given 

 by the paired parapodia, we find that there are twenty-one 

 somites present — a limited definite number which is also the 

 precise number found in all the higher Crustacea. 



We can state as a First Law ^ of metamerism or somite 

 formation that it is either indefinite in regard to number of 



' Tlic word "Law" is used in this suinniary merely as a coiiveuieut 

 lieading for the statement of a more or less general proposition. 



