Every one has pretty clear notions as to the general ‘make’ of a Vertebrate or backboned animal. An Invertebrate animal has, of course, no backbone or the semblance of one; the nervecord, where present, lies on the under surface, and forms a ring round the gullet, and the heart lies on the upper surface or back. We may verify this by pulling to pieces a dead insect.
But a phylum, or main division, is much too large to be considered as a whole. It must, therefore, be broken up into smaller groups, which are called Classes, generally reckoned as five in number. These, again, may be grouped into two divisions, according as their members breathe by means of air-tubes (tracheae) or by gills. Our scheme then will stand thus:—
| ARTHROPODS |  | Breathing by air-tubes | | Peripatus. |
| Centipedes and Millipedes. | ||||
| Insects. | ||||
| Spiders and their kin. | ||||
| Breathing by gills | Lobsters, Crabs, Sand-hoppers, and Woodlice. |
This scheme looks well on paper; and on the whole is workable. But among our examples chosen from the Class of Insects, we shall find some that breathe by gills in their larval stage, and by air-tubes when adult. And among the Crabs are some, the gills of which have ceased to perform their normal function, so that these animals cannot live in water for a single day. And then there are the Sand-hoppers and Woodlice.
The body of an Arthropod may be represented by a series of similar rings, thus:
This similarity is clearly apparent in the Centipede, but is concealed in the Beetle, the Shrimp, and the Spider. It seems, at first sight, to be altogether lost in the Crab, and does really vanish in the adult stage of some parasitic Crustaceans.
It may be plausibly objected that our ideal Arthropod resembles nothing so much as a worm. In many respects this is true. A primitive Arthropod was worm-like, as is a Centipede. And Arthropods and Worms were formerly classed together in one group, as Annulo´sa or ringed animals. The chief external difference lies in the nature of the appendages borne by the various rings or segments.
We may represent those of the Worms thus
