326 PHYLUM ARTHROPOD A. 



the numbers in the first column must, in our view, be regarded 

 as provisional. 



It will be seen that in Peripatus we have evidence of three 

 head segments, in the Crustacea and Myriapoda (though the 

 case of the Diplopoda is doubtful) of six. In the Insecta a 

 seventh segment, that of the labium, appears to have been 

 added, while in Arachnida the anterior part of the body, here 

 called the cephalo thorax, is apparently composed of eight 

 segments. 



The term acron occurring in the first and second columns of 

 the table was introduced by Janet for the region of the embryo 

 surrounding the mouth (as the telson surrounds the anus). It 

 is used here, following Heymons, in a more restricted sense for 

 the region in front of the mouth, containing as its median 

 element the clypeal shield, from which the labrum pro- 

 jects backwards over the mouth. From the epiblast of 

 the acron the syncerebrum of Scolopendra is developed. 

 Protocephalon is the name given by Heymons to the region 

 occupied by the procerebrum. The acron is regarded by 

 Heymons as the homologue of the prestomial lobe of Annelids. 

 This conclusion leaves out of consideration the case of Peripatus, 

 which from the distinctness of the coelomic sacs and the persist- 

 ence of nephridia cannot be disregarded in discussing the primi- 

 tive segmentation of Arthropods. It is difficult to believe, 

 notwithstanding the fact that the antennary somites of Peripatus 

 are postoral in origin, that mesoblastic structures have ever 

 existed in front of them, and if this is so these somites represent 

 the most anterior segment of the primitive Arthropods, whether 

 differentiated into prestomial lobe and peristomium or not so 

 differentiated. However, as stated in an earlier part of the 

 present work (Vol. I, p. 448), the relation of the prestomium to 

 the segments of the body is far from clear in the Annelids them- 

 selves, and we may here confine ourselves to pointing out that 

 the evidence remains conflicting when considered from the 

 point of view of Arthropods. 



We have to conclude then that the homology of the seg- 

 ments in Myriapods, Insects and Crustacea, though not free 

 from difficulty is fairly clear ; that the homology is obscure in 

 the case of Arachnids, and still more so in that of Peripatus. 

 The problem of the relationship between the anterior segments 



