Construct a dichotomous key for Chaetognatha families:
1. Family Spadellidae - Littoral, usually resting on algae, stones or other objects on the bottom, but retaining the ability of creeping and swimming. Tail segment is relatively large. One or two pairs of lateral fins. Anterior and posterior rows of teeth, which show a trend to the degeneration. Corona ciliata on the neck, wider than long.
2. Family Eukrohniidae - Pelagic, tail segment is comparatively short. One pair of lateral fins, each beginning far anterior to the trunk-tail septum. Anterior tooth-row or both anterior and posterior tooth-rows may be missing in some genera. So far as it is described, the conona ciliata begins at the posterior edge of the brain and ends at the neck region. All valid forms are known from the deep water at least in the warm-water regions.
3. Family Sagittidae - Two pairs of lateral fins.
4. Family Pterosagittidae - One pair of lateral fins, each beginning at the trunk-tail septum. The massive collarette is developed along the body sides, and there is a tuft of small tentacular protuberances on each lateral side at the level of the ventral ganglion.
5. Family Krohnittidae - One pair of lateral fins, each beginning above the trunk-tail septum. The shape of the tail fin differs slightly from that in other families.
6. Family Amiskwiidae - is proposed here for it, should be characterized by the location of the anus at the caudal end of the body. Other external features such as the existence of a pair of stout cephalic tentacles and others are regarded as being of the family or generic importance.
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