THE GROWTH OF JELLY-FISHES. 583 



contact with some solid body upon the bed of the ocean, it dies. 

 With the discovery of a solid resting-place its purpose is accom- 

 Xjlished : it loses its cilia, and, cementing its body fast, it elongates 

 and becomes converted into a root, from which the bud which is 

 to form the first feeding hydra soon arises, and, acquiring a mouth 

 and tentacles, begins to accumulate food and to provide for the 

 growth of a new colony. In some other species the planula be- 

 comes a feeding hydra instead of a root, and the history of the 

 various hydroids shows clearly that the root is directly compara- 

 ble with a hydra and is a member of the community which, like 

 the others, is specialized for a particular purpose. 



On a sea-beach there are few hard solid bodies except the 

 shells of mollusks, and these are therefore the only available rest- 

 ing-places for the planulse, but a colony which is founded upon 

 the shell of a living mollusk has no chance of prosperity, for the 

 mollusk is sure to soon plow its way under the sand or into the 

 mud, and a delicate hydroid can not survive such rough usage, 

 nor is the case of a planula which finds an empty shell any better, 

 for the first storm will either bury it under the sand, or toss it 

 high and dry above low-tide mark, or sweep it off into some deep 

 channel to be buried under the mud and sediment. 



Everything is favorable to the new colony which is started on 

 a shell which a hermit-crab has selected for a house, and the 

 chances are that it will grow and prosper and soon become a vig- 

 orous, flourishing settlement ; for the crab does not creep like a 

 snail, but trots around on the tips of his claws with the shell held 

 well up above the sand, and he is far too intelligent and wide- 

 awake to permit himself to be stranded on the beach, or swept 

 away into muddy quicksands. As the gentle waves ebb and flow 

 on the shore he follows them back and forth, keeping close to the 

 edge, where the food which is washed out of the sand is most 

 abundant and the aeration of the water most perfect. As long as 

 the sea is calm he may be trusted to carry his load of hydroids 

 into the places which are most favorable for them, and as soon 

 as a storm approaches he trots off with his charge to a safe 

 shelter in deeper water and waits until it has passed. A colony 

 which is founded on his shell is sure to flourish and increase, 

 for this location affords all the elements of prosperity, and, while 

 small colonies are often found in other places, the most vig- 

 orous and largest ones are, as a rule, found only in this peculiar 

 habitat. 



In the following diagram I have attempted to exhibit at one 

 view all the phases in the remarkable life-history of Dysmorphosa. 

 The sign of equality = between two stages indicates that the one 

 on the left becomes transformed into the one on the right, without 

 multiplication and without loss of identity ; the sign x indicates 



