The digestive tube resembles that of the Chilopoda. The legs are very long and slender, and the joints are beset with bristles. Both sexes have small hook-like appendages at the sides of the genital openings.
The eyes have already been mentioned as being more highly developed than in other classes, in correspondence with the more active habits of the animal. The generative organs open at the hind end of the body, as in Chilopoda.
The heart is highly developed, quite as much so as the Chilopod heart, the alary muscles being strong and broad, and the arteries being quite as perfect as those in any Myriapod. The muscular coats which govern the pulsations by their contractions are powerful and well developed.
Order IV. Symphyla.
We next come to one of the last two Orders which have been recently added to the Myriapoda. These little animals have a great resemblance to the Thysanura among the Insects, and especially to Campodea among the Thysanura. It will be well, therefore, to begin our account with a few of the reasons which have induced naturalists to include them among the Myriapods rather than among the Thysanura.
1. Campodea has three pairs of mouth appendages, while Scolopendrella has only two.
2. Scolopendrella has broad plates covering the back, not only on the anterior (thoracic) segments, but on the whole body.
3. The terminal appendages of Scolopendrella differ from those in Campodea.
4. Scolopendrella has a sense organ which is absent in Campodea.
5. Campodea breathes by means of three stigmata in the anterior part of the body. The stigmata of Scolopendrella are hard to see, and are not in the same position.