1. Under standard conditions Peripatopsis loses water twice as rapidly as an earthworm, forty times as rapidly as a smooth-skinned caterpillar, and eighty times as rapidly as a cockroach.

2. Water loss almost certainly takes place through the tracheae.

3. Lack of control of water loss is due to the large numbers of spiracles which have no closing mechanism.

4. Although the Onychophora have many efficient adaptations for a terrestrial habit, the type of tracheal system alone may have been responsible for the group not becoming widespread and successful.

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