THE MYRIArODA OF NORTH AMERICA. 



141 



but the mode of development and growth of the latter is so different from that of the 

 hexapoda, arachnida or octopoda, and Crustacea or decapoda, that it seems to me they 

 must be acknowledged as a separate class. Now, their method of development is similar 

 to that of the Vermes, hence the reason of their being placed next above them. When a 

 spider, or insect, or crustacean leaves the egg, its body has its maximum number of seg- 

 ments, and development takes place by the coalescence and disappearance of some of these. 

 The embryonic myriapod, on the contrary, has its minimum number of segments, and 

 develops by their increase. So that whilst the adult insect has generally fewer, never 

 more segments than the young, the adult myriapod may have eight times as many, and 

 never fewer than its young.* 



Ord. I. CHILOPODA. Leach.j 



Corporis segmenta, singulum pedum pare unieo instructum. 

 Segments of the body, each furnished with a single pair of feet. 



The body in the Chilopoda is composed of segments, whose number varies from sixteen 

 to several hundred. Each segment is furnished with a single pair of legs. There might 

 seem to be an exception to this in the Cermatiida?, but close examination shows that in 

 them the fact of there being but one scutum to two segments is the cause of the apparent 

 anomaly. In all the other families there is a scutum and sternum to each segment. These 

 external bones, so to speak, are connected by a tough membrane, which forms the only 

 protection to the sides. In the upper portions of this are placed the spiracles, in the lower 

 the insertion of the legs. 



Close to the spiracles, and belonging to the dorsal section, are two osseous points, the 

 rudiments of the paraptera, which at- Fig. 2. 



tain to some importance among the 

 Hexapoda. At the insertion of the legs 



are several small plates, the epimera c 



(Fig. 2, c) , which afford points of origin 

 to the retractor muscles of the legs. 

 The posterior of the two embryonic sub- 

 segments forms the mass of the segment ; 

 but the scutum of the anterior is perhaps represented by a raised band on the front of the 

 main scutum. The ventral portions of it are, however, much more distinct. These are 



* I would refer any one desiring to follow this further to Mr. Newport's paper. Linn. Trans., vol. xix. 

 f Linn. Trans., vol. xi. 



