THE HIGHER GROUPS. 573 



There is no invertebrated animal at present known which 

 cannot at once be referred to one or other of the natural or- 

 ders which have been discussed in the preceding pages. The 

 next question which arises is, How far are these groups sus- 

 ceptible of arrangement into assemblages of a higher order, 

 distinguished from all others by certain common characters ? 



It is universally admitted that the Insecta, Myriapoda^ 

 Arachnida^ Crustacea, Pycnogonida, and Tardigrada, form 

 such an assemblage, termed the Arthropoda, and character- 

 ized by the segmentation of the body ; the chitinous cuticula ; 

 the absence of cilia upon, or in, the body at any period of life ; 

 the segmentation of the central nervous system, and its per- 

 foration by the gullet ; and the presence (with the possible 

 exception of the Irllohlti) of limbs, which, almost always, 

 are themselves subdivided into joints. The reasons for in- 

 cluding the Per ipat Idea in this division have been given in 

 Chapter XL ; and, though the Pentastomida must be regarded 

 as hardly within the limits of the definition, I think that, tak- 

 ing into account the strange modifications which are under- 

 gone by the parasitic Crustacea and AracJuiida, it is not 

 needful to depart from the ordinary practice of associating 

 them with the Arthropoda. 



The LimelUhranckiata and the Odontophora constitute 

 another very well marked division, the Mollusca, the char- 

 acters of which have been discussed in Chapter VIII. 



The proposal to separate the Polyplacophora from the 

 3Iollusca, to which I have already referred, appears to me to 

 be devoid of any justification. The resemblances between 

 certain Gephyrea, such as ChcModenna and Neomenla, and 

 the Polyplacophora, are accompanied by wide diff'erences ; 

 and even if these resemblances are to be regarded as evi- 

 dences of affinity, some considerations, such as the restriction 

 of the branchiae to the hinder part of the body, and the reduc- 

 tion of the foot in Chitonellus, rather lead to the suggestion 

 that Chmtoderma and Neomenia may be extremely modified 

 MoUusks, allied to the Polyplacophora. 



As to the supposition that the resemblances between the 

 Nudlhranchlata and the Tarbellaria indicate a direct affin- 

 ity between these groups, it seems to be forgotten that the 

 N^udlhranchiata are all, when young, unmistakable Gastero- 

 pods provided with mantle and shell. Their adult structure 

 is as little evidence of any Turbellarian affinities as that of 

 Lerucea is proof of its being allied to the Avorms rather than 

 to the Crustacea. 



