250 EDWTN^ S. GOODRICH. 



longitudinal nerve-cords, and (in Polychsetes and Arthropods) 

 a pair of appendages. But we know very well that such fully 

 equipped segments are rarely found in nature. Intersegmental 

 grooves frequently disappear (head and thorax of Arthropods) j 

 there are segments without ccBlomic cavity (the thoracic seg- 

 ments of insects, for instance) ; and again, there are segments 

 with neither peritoneal funnels nor nephridia (most Arthro- 

 pod and Chsetopod anterior segments). Some metameres 

 have no ventral nerve-cord (the first two segments of Lum- 

 bricus, the posterior abdominal segments of many insects) ; 

 appendages are often absent. 



It is clear, then, that the examination of adult structure 

 will help us little in deciding whether a debatable region re- 

 presents a true segment or not, though a careful comparison 

 with allied types may often be of use. Embryology is our 

 best guide in these cases, and generally furnishes a decisive 

 answer. We find, as a matter of fact, that the segments, 

 which lack in the adult those structures most essential, possess 

 them at some time during early development, and lose them 

 at a later period. Yet here, again, it must be admitted that 

 undoubted metameres may have lost even during development 

 one or more of the structures characteristic of true segments 

 (for instance, no distinct ccelomic cavity is known to occur 

 in some of the anterior segments of many Crustacea). We 

 cannot, therefore, assert that a given region is not a metamere 

 because it does not possess this or that character. The only 

 dogmatic statement we are justified in making is, that when a 

 region exhibits during development a sufficient number of the 

 essential structures of a typical segment, it may be assumed 

 to be a true metamere.^ What is "sufficient" has to be de- 

 cided in each case. It should be added that one positive fact 

 outweighs many negative ones ; the known presence of a certain 

 characteristic of a segment in a certain region of an Arthropod, 

 for instance, is of far greater importance than its ascertained 

 absence in numerous other cases. A good example of this 



' Theories as to the origin of metameric segmentation do not concern us 

 here ; at any rate, I do not propose to include them in this discussion. 



