STRUCTURE AND CLASSIFICATION OF THE AETHROPODA. 533 



Arthropod body to rely exclusively upon the indications ^iven 

 by the tegumentary chitinons plates and the parapodia. 



The Third Law of metamerism is that heteromerism may 

 operate in such a way as to produce definite regions of like 

 modification of the somites and their appendages, differing 

 in their modification from that observed in regions before and 

 behind them. It is convenient to have a special word for 

 such regions of like meres, and we call each a tagma {rayfia, 

 a regiment). The word " tagmosis " is applicable to the 

 formation of sucli regions. In the Chjetopods tagmosis 

 always occurs to a small extent, so as to form the head. lu 

 some Chsetopods, such as Chaetopterus and the sedentary 

 forms, there is marked tagmosis, giving rise to three or even 

 more tagmata. In Arthropods, besides the head, we find 

 very frequently other tagmata developed. But it is to be 

 noted that in the higher members of each great class or line 

 of descent, the tagmosis becomes definite and characteristic 

 just as do the total number of meres or somites, whilst in the 

 lower grades of each great class we find what may be regarded 

 as varying examples of tentative tagmosis. The terms nomo- 

 tagmic and anomotagmic are applicable with the same kind 

 of implication as the terms nomomeristic and anomomeristic. 



The FouBTH Law of metamerism (auto-heterosis of the 

 merouies) is that the meromes of a somite or series of somites 

 may be separately and dissirailarlj' affected by heteromerism. 

 It is common enough for small changes only to occur in the 

 inner visceral meromes, whilst the appendages and terga or 

 sterna are largely changed in form. But of equal importance 

 is the independent "heterosis" of these visceral meromes 

 without any corresponding heterosis of the body-wall. As 

 instances we may cite the gizzards of various earthworms, 

 and the special localisation of renal, genital, and gastric 

 meromes, with obliteration elsewhere, in a few somites in 

 Arthropoda. 



The Fifth Law, relating also to the independence of the 

 meromes as compared with the whole somite, is the law of 

 autorhythmus of the meromes. Metamerism does not always 



