Page:Wood 1865 - The Myriapoda of North America.djvu/6

Rh 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 segments, 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.

. CHILOPODA. Leach. Corporis segnienta, singulum pedum pare unico 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 Cermatiidæ, 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 attain to some importance among the Hexapoda. At the insertion of the legs are several small plates, the epimera (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