280 Mr. Newport on the Class Myriapoda, Order Cbilopoda, 



lata the original division of the skeleton into segments is less apparent than 



in the Myriapoda, in which each segment remains distinct throughout life. 



First, then, 



Of the Structure of the Segments. 



Every segment of the body, in its normal state, is formed of two arched sur- 

 faces, a dorsal and a ventral. Each of these surfaces is originally developed 

 in four parts, two of which are situated on each side of the median line. The 

 two middle ones of the four portions on the ventral surface (Tab. XXXIII. 

 figs. 1 and 2) unite very early to form the single middle portion of the ventral 

 plate (a), and together constitute the single median portion or true sternum. 

 The two lateral pieces (b) are united to the sternal at nearly the same period, 

 and constitute the episternal plates, and the whole form one large, broad, 

 shield-shaped, abdominal plate to each segment, as is well seen in Scolopendra, 

 in which genus the two arches of each segment are more uniformly developed 

 than in any other. The longitudinal line of union of the two sternal pieces 

 (a a) entirely disappears, but that of the episternal (b) with the sternal remains 

 permanent throughout life as a longitudinal sulcus. This is more especially 

 marked at the anterior border of each plate. 



The dorsal arc (fig. 4.), composed like the ventral of four pieces, is less early 

 completed. The two median plates are the last to be united. They form the 

 scutum (d) or upper surface of each segment, along the middle of which there is 

 usually an indication of its original longitudinal line of separation. This 

 longitudinal line, or depression, corresponds internally to a longitudinal line of 

 attachment for the median suspensory muscles of the heart, or so-called dorsal 

 vessel, in each segment. The two lateral portions of the dorsal arc are united 

 longitudinally with the middle ones, and form the eplscuta (e), the whole to- 

 gether constituting one large dorsal plate or shield. The relative development 

 of the two arcs of each segment is carried to the greatest extent in all the 

 Myriapoda in the dorsal region, the episcutal plates being always more ex- 

 tensively developed than the episternal. 



These are the normal structures in each segment. But the order in which 

 the individual parts are completed on the dorsal surface is the reverse of those 

 on the ventral. This difference appears to have some reference to the rela- 

 tive position of the nervous and vascular systems. The nervous cord in the 



