STRUCTURE AND CLASSIFICATION OP THE ARTHROPODA. 537 



two or more somites hj g-rowth and gradual elabornti'^n — 

 wliat is called " budding '^ — of tlie anterior border of the 

 hindermost somite. This hindermost somite is therefore 

 different from all the other somites, and is called the Helsou.' 

 However long or short or heteromeriscd the chain may be, 

 new metameres or somites are only produced at the anterior 

 border of the telson, except in the Vertebrata." That is the 

 general law; but amongst some groups of metamerised 

 animals partial exceptions to it occur. It is probably abso- 

 lutely true for the Arthropoda from lowest to highest. It is 

 not so certain that it is true for the Ch^etopoda, and would 

 need modification in statement to meet the cases of fissi- 

 parous multiplication occurring among Syllids and Naidids. 

 In the Yertebrata, where tagmosis and heterosis of meromes 

 and dislocation of meromes and tagmata are, so to speak, 

 rampant, new formation of metameres (at any rate as repre- 

 sented by important meromes) takes place at more than one 

 point in the chain. Such points are found where two highly 

 diverse "tagmata" abut on one another. It is possible, 

 though the evidence at present is entirely against the supposi- 

 tion, that at such points in Arthropoda new somites may be 

 formed.^ Such new somites are said to be "intercalated." 

 The question of the intercalation of vertebras in the Verte- 

 brata has received some attention. It must be remembered 

 that a vertebra, even taken with its muscular, vascular, and 

 neural accessories, is only a partial metamere — a merome, and 

 that, so far as complete metameres are concerned, the 



* Tlie curious case of superabundant parapodia in tlie hinder somites of 

 Apus has already been cited and referred to as an example of autoibytiimic 

 multiplication of meromes. There is some reason for regardin;^ the extra 

 pairs of legs as being "intercalated" after the formation of the somite as a 

 single unit or merome by growth from the telson. Supposing, as appears to 

 be the case, that as the Apus increases iu size, the number of extra pairs of 

 legs on a non-terminal somite increases, these added meromes are certainly 

 intercalated, and represent incomplete intercalated metameres. The intercala- 

 tion of new elements does not really go much further than this in Vertebrata, 

 for a vertebra with its myoskeietal tissues is only a merome, and not a comj)lete 

 metamere. 



VOL. 47, FART 4. — NEW SERIES. M M 



