NO. 3 INSECT HEAD SNODGRASS I 5 



The component segments of the cephalic loljes, or head of the arthro- 

 pod embryo, are never distinct, but the subsequent development of 

 the anterior nerve centers shows that the lobes comprise two segments 

 at least, in addition to a prostomial region, and that usually a third 

 segment is more or less included in their posterior part. 



The way in which metamerism arose in the phylogenetic history of 

 segmented animals is not known, and it is not necessary to believe 

 that the method of segment formation in either the annelid larva or 

 the arthropod embryo gives a picture of primitive segmentation in 

 the course of evolution. The development of the trochophore into the 

 worm is clearly a process of metamorphosis, that is, it is the return 

 of a specialized, aberrant larva to the ancestral form represented more 

 nearly in that of the adult ; and it is well known that embryos do not 

 keep closely to the phylogenetic path in the details of their development. 

 Since so many other essential features in the body structure of animals 

 are connected with the mode of locomotion, the writer holds as most 

 probable the idea that segmentation also had its beginning as an 

 adaptation to a specific kind of movement. The creeping, worm-like 

 ancestors of the annelids and arthropods certainly at an early period 

 must have developed a contractile tissue in their mesoderm bands — - 

 that they did so is attested by the early development of a central 

 nervous system consisting of motor neurons, following the lines of the 

 later established ventral longitudinal muscle bands. It is, then, clear 

 that a breaking up of the contractile tissue into short lengths would 

 give a greater efficiency of movement, with the possibility of more 

 variety of action, and that, wdth the differentiation of true muscle 

 fibers, the attachment of the ends of the fibers to the ectoderm would 

 carry the metamerism into the body wall. The fact that embryonic 

 segmentation begins anteriorly and progresses backward, in itself 

 suggests that metamerism originated in a creeping animal ; in a free- 

 swimming form, the progress of segmentation should be the reverse, 

 for the motile region of the animal would be the tail end. Organs de- 

 veloped at the time of metamerism or subsequent to it, such as ne- 

 phridia, tracheae, and external appendages, are repeated in each seg- 

 ment, those antedating segmentation either remain unsegmented, as 

 the alimentary canal, or take on a secondary segmentation, as do the 

 body wall and the nervous system. 



There are other theories of metamerism: Hatschek (i 888-1 891) 

 enumerates five views that have been proposed to explain the origin 

 of body segmentation, but none of them is based on the simple fact 

 that in embryonic development, metamerism begins in the mesoderm 



