Page:Wood 1865 - The Myriapoda of North America.djvu/3

138 by the atrophy of one and the hypertrophy of the other. The dorsal and ventral portions 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 episterna, unite sooner or later. Well-pronounced 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. 1. b), each with its episcutum (Fig. 1, a); and two primitive sterna (Fig. 1, c), each with its episternum (Fig. 1, d). 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 integument. 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 membrane, 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 are 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 furnished 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,