254 EDWIN S. GOODRICH. 



stomiura, being neither a reduced nor an incipient segment^ is 

 a special region not of segmental value. Further, we may 

 take two views of this question : the first, and the one 

 generally held, is that the prostomium represents the region 

 lying in front of the mouth of the primitive unsegmented 

 ancestor; the second is that the prostomium is a new growth 

 from the first segment, or region surrounding a terminal 

 mouth in the primitive ancestor.i According to the first, the 

 prostomium is a region of great morphological and phylogenic 

 importance. According to the second, a more recent addition 

 of relatively little significance. A comparison of the merits 

 of these two views would land us in the midst of theories into 

 which there is no need to enter here ; it is sufficient for our 

 present purpose to have shown that the prostomium is not a 

 true segment. 



In the Arthropoda we find a region in front of the mouth 

 of varying size, bearing as a rule antennse and well-developed 

 sense-organs, and containing the brain. A certain number 

 of segments behind bear appendages connected with and 

 modified in relation to the mouth. These, together with the 

 preoral region, constitute the " head." Long ago the evidence 

 of comparative anatomy and embryology convinced naturalists 

 that the head of Arthropods, both in front and behind the 

 mouth, is composed of several metameres, more or less fused 

 together. It is with the preoral region that we are most 

 directly concerned in our comparison with the Annelid. 



How do true segments come to lie in a preoral position ? 



is one of the first questions we have to answer. If the first 



1 Sucli a supposition would lead us, perhaps, to somewhat modify our con- 

 ception of the peristomium (first segment) as being merely a metamere, 

 since it would have the property of producing an anterior prostomial out- 

 growth (and brain). It must be remembered, however, that, in the cases of 

 reproduction by fission, other and posterior segments possess the same power. 

 Lankester has assumed that theoretically every segment should develop a 

 prostomium, and is only as it were withheld from this completion of itself by 

 a " longitudinal cohesion or integration." " In most Annulosa this longitudinal 

 cohesion counteracts entirely the opposing tendency to produce a head and to 

 separate " (8). 



