138 



THE MYRIArODA OF NORTH AMERICA. 



Fig. 1. 



T 



by the atrophy of one and the hypertrophy of the other. The dorsal and ventral por- 

 tions of the subsegments are each formed originally from four pieces. The two central 

 unite very early in the embryo, and completely fusing form the primitive scuta and sterna, 

 with which the outer pieces, the episcuta and epistcrna, unite sooner or later. Well-pro- 

 nounced grooves or sutures in the adult frequently indicate the point of union between 

 the epi and primitive sterna, the epi and primitive scuta. A typical or ideal segmental 

 arc (Fig. 1) would therefore be formed by the conjunction and coalescence on the dorsal 



surface of four pieces, and of the same number on 

 the ventral aspect : i. e. } two primitive scuta (Fig. 

 I, h), each with its cpiscutum (Fig. 1, a); and two 

 primitive sterna (Fig. 1 , c) , each with its epistcrnum 

 (Fig. 1, <l). To the upper or dorsal portion pertains 

 the organs of vegetative life: i. e., the circulatory 

 and secretory apparatuses ; to the lower or ventral portion the organs of animal life, the 

 nervous system. 



In this typical segmental arc the perfected scutum and sternum do not coalesce, nor do 

 they even approximate and unite by suture, but are connected by a'strong flexible integu- 

 ment. This membrane is divisible into two regions, the upper of which is subsidiary 

 to the scutum, the lower to the sternum. Now, it is in this portion of the segmental arc 

 that the appendages are formed, a sternal and scutal pair to each subsegment. On the 

 examination of a Scolopendra, some little plates will be found situated on the lateral mem- 

 brane, just anterior to the spiracles. These are the rudiments of the scutal appendages, 

 which' are never more highly developed in the Myriapoda. The normal scutal appendages 

 of an articulate arc wings, whose points of origin in the hexapoda correspond to these 

 plates. Of course, then, in apterous articulates they cannot be developed. The sternal 

 appendages are the organs of locomotion, the legs. To sum up, a typical myriapodal 

 segment is formed from two subsegments, each of which is originally formed from eight 

 pieces, and is composed of a dorsal and ventral arc, united by a membrane, and fur- 

 nished with a single pair of appendages. 



The existent myriapodal segments follow two structural plans or types, modifications of 

 the typical or ideal. These constitute the two great orders, the Chilopoda and the Diplopoda. 

 In the first of these, the anterior subsegment is so atrophied that it really forms no part 

 of the perfected whole. Its appendages are entirely lost ; indeed, its only representatives 

 are some small plates, situated on the ventral surface just anterior to the sterna. On the 

 other hand, the posterior subsegment, with its appendages, is developed to the highest 

 degree, and equally, or nearly equally so, in all its parts, neither the ventral nor dorsal 

 portion of the arc predominating. So in a Chilopod there are large sterna and scuta, sepa- 



