STUUCTUKE AND CLASSIFICATION OF TBE ARTHROPODA. 543 



(2) of the backward movement of the mouth in Arfchropoda 

 from the first somite to the second, third, or even fourth in 

 the original post-oral series, is not only justified by embryo- 

 logical observation of the shifting in question, but finds 

 its parallel in other instances of the law of dislocation of 

 meromes. 



The fusion of the cephalic or prosomatic somites not only 

 extends to tegumentary structures, but to muscles, blood- 

 vessels, and markedly to neuromeres. However, in the 

 embryo of many Arthropoda the original neuromeres of the 

 pr^eoral somites can be distinguished, and in many cases 

 the coelomic cavities. Also it is a noteworthy fact that the 

 tegumentary fusion (cephalic carapace, prosomatic carapace) 

 appears sometimes to break down secondarily (e. g. Squilla 

 among Crustacea and Galeodes and Tarassidte among Arach- 

 nida). It appears that we must recognise as a principle 

 that such fusions as the carapaces of Arthropoda can revert 

 to the condition of free movable plates ; and therefore we 

 must not assume that forms with fused tergal plates are 

 necessarily later, genetically, than allied forms with free 

 movable tergal plates. 



When such reversion to a movable series of dorsal plates 

 occurs it must not be assumed that any corresponding change 

 takes place in the deeper meromes. On the whole, fusion 

 and ankylosis of somites is not in itself necessarily a deep- 

 seated or far-reaching process. It may or may not be 

 accompanied by dislocation of important meromes or by 

 lipomerism ; whilst, — as for instance in the opisthosoma of the 

 spiders, opiliones, and acari — dislocation and lipomerism may 

 occur without fusion of tegumentary plates, and with, on 

 the contrary, a dwindling and eventual atrophy of such 

 plates. 



The general considerations as to metamerism set forth 

 above will enable us to proceed to a consideration of the 

 characters which distinguish the various groups of Arthro- 

 poda, and to justify the classification with which we started. 



The Theory of the Arthropod Head. — The arthropod 



