RELATION OF ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 255 



segment behind the moutli in Arthropods represent the peri- 

 stoniium in Annelids, then those between it and the anterior 

 extremity must be new metameres — a supposition whieh, 

 if true, would contradict tiie general law of segmentation and 

 the evidence of ontogeny.^ Professor Lankester in 1873 

 argued that these preoral segments must originally have been 

 post-oral, and that they have since moved forwards in front of 

 the mouth — or, in other words, that the mouth has shifted 

 backwards. 2 This fertile suggestion is supported by the facts 

 observed in the ontogeny of Arthropods generally ; and even 

 in the Polychpeta there is often a tendency for the primitively 

 post-oral segments to shift forwards in front of the mouth, 

 as in many Amphinomids and in Aphrodite. Lankester's 

 explanation has been generally adopted, the only difference of 

 opinion being as to how many of the "head segments" are 

 true metameres, and therefore of post-oral origin. 



Of the highest importance in connection with this problem 

 is the study of the structure and development of the brain. 

 It is well known that the Arthropod brain presents the 

 appearance, either in the embryonic or adult state, of being 

 formed of several segments. Here again the suggestion made 



* Segments develop from before backwards. Although the sequence of the 

 differentiation of tlie anterior segments of Arthropods may be somewhat 

 obscured by what is ahnost certainly secondary modification and retardation 

 (cheliceral segment in Arachnids), yet we never find the germ bands, after 

 the first segment has been formed, growing forwards beyond it to give rise to 

 pew segments in front. 



^ " The segmentation of the prostomial axis in Arthropoda and some Anne- 

 lids, which has an appearance of being a zooid segmentation comparable to 

 that of the metastomial axis, on account of the identity in tiie character of 

 the appendages with tliose of the metastomial axis, has yet to be explained. 

 It may be suggested that it is due to a distinct breaking up of this axis like 

 the posterior one into zooid segments or zoonites ; there is much against this 

 supposition (see ' Trans. Linn. Soc.,' 1869, " On Chsetogaster and J]]olo- 

 soma"). Much more likely, it seems, is the explanation that the oral aper- 

 ture shifts position, and that the ophthalmic segment alone in Arthropoda 

 represents the prostomium, the auteuuary and auteunular segments being 

 aboriginally metastomial, and only prostomial by later adaptational 

 shifting of the oral aperture" (9). 



VOL. 40, PART 2. NEW SER. T 



