THE JOINTED-FOOT ANIMALS 139 



but since its enemies are probably quite as persistent there as 

 in the shallow water, every means of defense is likely to be 

 employed to the utmost. 



The lobster is capable of swimming with great rapidity, 

 by suddenly doubling its flexible, muscular abdomen beneath 

 and shooting backward through the water. The habit does 

 not appear to be depended on except to enable them to escape 

 from impending danger. Their usual mode of progression is 

 by walking on the tips of the last four pairs of thoracic 

 appendages, the chelipeds being extended anteriorly, appar- 

 ently to expose as little of their bulk to the water as possible. 

 The swimmerets waving in rhythmic motion aid the animal 

 in its movement along the sea-bottom. 



With all their appearance of armored strength, lobsters are 

 very sensitive to changed conditions in their environment. 

 When captured and detained in "pounds " (artificially inclosed 

 spaces), in shallow water, they will, on the approach of win- 

 ter, dig burrows in the sea-bottom and cover themselves all 

 but their eyes and antennae. There they remain in a dor- 

 mant condition until the returning warmth of spring affects 

 the temperature of the water. In their natural environment 

 they migrate to the deep waters in the autumn months and 

 remain there till spring. We know that they do not make 

 burrows and lie inactive in the deep waters, because fishermen 

 catch them in baited traps in all the winter months. In the 

 winter in our latitude deep waters are warmer than shallow 

 waters; in summer the shallow waters are warmer. Lobsters 

 are sensitive to even slight changes in temperature. In the 

 early summer they have been known to return to deep water 

 immediately on the occurrence of a storm in the atmosphere 

 above them. Another factor which must operate strongly in 

 determining the migration to the shore is the greater supply 

 of food there. Lobsters do not appear to migrate up and 

 down the coast. Hence, if all the lobsters in a certain bay 



