A student found a small animal attached to the skin of a dog. Under a hand lens it showed: a hard jointed exoskeleton; jointed legs;
four pairs of legs in the adult;
no antennae; mouthparts adapted for piercing skin and sucking blood; and the head and thorax fused into a single region joined to the abdomen. Use the dichotomous key below.
| Couplet |
Feature |
Outcome |
| 1a |
Jointed appendages and a jointed exoskeleton present |
go to 2 |
| 1b |
No jointed appendages |
go to 5 |
| 2a |
Three pairs of legs and one pair of antennae |
Insecta |
| 2b |
Not three pairs of legs, or antennae absent or more than one pair |
go to 3 |
| 3a |
Four pairs of legs; no antennae; body in two regions (cephalothorax and abdomen) |
Arachnida |
| 3b |
More than four pairs of legs |
go to 4 |
| 4a |
One or two pairs of legs on almost every body segment; one pair of antennae |
Myriapoda |
| 4b |
Two pairs of antennae; exoskeleton hardened with calcium carbonate; breathes by gills |
Crustacea |
| 5a |
Soft, unsegmented body, often within a shell; muscular foot |
Mollusca |
| 5b |
Body divided into ring-like segments bearing chaetae |
Annelida |
Which option gives the correct path through the key and the group to which the animal belongs?
Select one option.