LEE BARKER WALTON 161 



notopodium) and a ventral branch {nr = neuropodium) . 

 There is also a dorsal appendage {dc = dorsal cirrus), 

 generally developed as a sensory organ, but often forming 

 a gill or more rarely a protective sheath called an elytron. 

 Occasionally other dorsal as well as ventral cirri, usually 

 of smaller size, are present. The dorsal and ventral 

 branches of the parapodium are each supported by a 

 large thick seta {ac = aciculum) enclosed in an integu- 

 mentary sack {as = acicular sack) , while groups of small 

 setae {st = setae) are found at the distal extremities of 

 both the notopodium and neuropodium. The openings 

 of epidermal glands {ig = integumentary gland) are 

 scattered over the surface as outlets for the integumentary 

 glands. 



There are only a few changes that need to take place in 

 the position and form of the parapodium to convert it 

 into the typical structure of the appendage with the 

 pleurites of the insect. These changes may be indicated 

 by a hypothetical segment intermediate in form between 

 the segment of a typical Polychaete and a typical insect. 

 The parapodium has moved ventrally and forms a plane 

 which in its relation to the axis of the body has turned 

 clockwise nearly forty-five degrees. The lower branch 

 {nr = neuropodium) has become large and somewhat 

 elongated as compared with the upper branch {nf = 

 notopodium) which remains short and small. The large 

 cirrus {dc =" dorsal cirrus) retains its development, while 

 any ventral cirri that may have been present have disap- 

 peared, unless we assume the retention of small basal cirri 

 which eventually developed as the propteron and pteron 

 (wing). The integumentary glands {ig = integumentary 

 gland) assume an increased size and function correlated 

 with the semi-aquatic environment of the organism and 

 become restricted in number. 



It is a result quite consistent with the ecological con- 

 ditions under which many of the Polychaetes existed in 

 marshy areas of the coastal regions. The parapodia would 

 more and more be compelled to support the body of the 

 individual as it moved through the shallow and partially 

 dried pools. The subsequent changes, including that of 



