90 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



lacinia and galea overlap each other basally, and both are flexed by 

 the muscles inserted upon their bases that have their origin in the 

 stipes. 



It is impossible at present to arrive at final conclusions on the many 

 problems connected with the morphology of arthropod appendages, 

 and the most that the writer would claim for the present attempt at 

 advancing the subject is that the material here presented gives at 

 least a substantial enlargement to the foundation of known facts from 

 which future work must proceed. There is no question that students 

 of arthropods have given far too little attention to the relationship 

 between skeletal structure and musculature. The more the svibject is 

 looked into, the more it will be seen that the characters of the arthropod 

 skeleton are in large part adaptations to the strain of muscle tension, 

 and that they are to be correctly interpreted only through an under- 

 standing of the entire mechanism of which they are a part. The 

 sclerites of the insect cuticula, in particular, have been studied as if 

 they were skeletal elements deftly fitted together in such a manner as 

 to cover the outside of the animal, and we entomologists have played 

 with them, as we might with the sections of a picture puzzle, without 

 looking for their significance in the mechanics of the insect. The 

 arthropod skeleton, it is true, has been formed from a few major 

 centers of increased chitinization, but the minor " divisions " are in 

 almost all cases adaptations to flexion, or the opposite, namely, the 

 strengthening of the skeleton by the development of internal ridges. 

 The scientific study of the comparative anatomy of insects must look 

 for its advance in the future to a wider knowledge of muscles and 

 mechanism. 



IV. SUMMARY OF IMPORTANT POINTS 



I. The arthropods have been derived from creeping animals, not 

 from forms specially modified for swimming ; their immediate pro- 

 genitors were annelid-like in structure. 



^2. The stomodeum marks the anterior end of the blastopore. There 

 are, therefore, no true mesodermal segments anterior to the mouth. 

 The unsegmented preoral part of the animal is the prostomium, and 

 constitutes the most primitive head, or archkephalon, of segmented 

 animals, since it contains the first nerve center, or " archicerebrum," 

 and bears the primitive sense organs. 



3. The first stage in the development of a composite head in the 

 arthropods, as represented in the embryo, comprises the prostomium 

 and the first two or three postoral segments. The head in this stage 

 may be termed the protocephalon; it is represented by the cephalic 

 lobes of the embryo, which may or may not include the third segment. 





